GIT\-FAST\-IMPORTSection: Git Manual (1)
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NAMEgit-fast-import - Backend for fast Git data importers
SYNOPSISfrontend | git-fast-import [options]
DESCRIPTIONThis program is usually not what the end user wants to run directly. Most end users want to use one of the existing frontend programs, which parses a specific type of foreign source and feeds the contents stored there to git-fast-import.
fast-import reads a mixed command/data stream from standard input and writes one or more packfiles directly into the current repository. When EOF is received on standard input, fast import writes out updated branch and tag refs, fully updating the current repository with the newly imported data.
The fast-import backend itself can import into an empty repository (one that has already been initialized by git-init(1)) or incrementally update an existing populated repository. Whether or not incremental imports are supported from a particular foreign source depends on the frontend program in use.
- Specify the type of dates the frontend will supply to fast-import within author, committer and tagger commands. See lqDate Formatsrq below for details about which formats are supported, and their syntax.
- Force updating modified existing branches, even if doing so would cause commits to be lost (as the new commit does not contain the old commit).
- Maximum size of each output packfile, expressed in MiB. The default is 4096 (4 GiB) as that is the maximum allowed packfile size (due to file format limitations). Some importers may wish to lower this, such as to ensure the resulting packfiles fit on CDs.
- Maximum delta depth, for blob and tree deltification. Default is 10.
- Maximum number of branches to maintain active at once. See lqMemory Utilizationrq below for details. Default is 5.
- Dumps the internal marks table to <file> when complete. Marks are written one per line as :markid SHA-1. Frontends can use this file to validate imports after they have been completed, or to save the marks table across incremental runs. As <file> is only opened and truncated at checkpoint (or completion) the same path can also be safely given to --import-marks.
- Before processing any input, load the marks specified in <file>. The input file must exist, must be readable, and must use the same format as produced by --export-marks. Multiple options may be supplied to import more than one set of marks. If a mark is defined to different values, the last file wins.
- After creating a packfile, print a line of data to <file> listing the filename of the packfile and the last commit on each branch that was written to that packfile. This information may be useful after importing projects whose total object set exceeds the 4 GiB packfile limit, as these commits can be used as edge points during calls to git-pack-objects(1).
- Disable all non-fatal output, making fast-import silent when it is successful. This option disables the output shown by --stats.
- Display some basic statistics about the objects fast-import has created, the packfiles they were stored into, and the memory used by fast-import during this run. Showing this output is currently the default, but can be disabled with --quiet.
PERFORMANCEThe design of fast-import allows it to import large projects in a minimum amount of memory usage and processing time. Assuming the frontend is able to keep up with fast-import and feed it a constant stream of data, import times for projects holding 10+ years of history and containing 100,000+ individual commits are generally completed in just 1-2 hours on quite modest (~$2,000 USD) hardware.
Most bottlenecks appear to be in foreign source data access (the source just cannot extract revisions fast enough) or disk IO (fast-import writes as fast as the disk will take the data). Imports will run faster if the source data is stored on a different drive than the destination Git repository (due to less IO contention).
DEVELOPMENT COSTA typical frontend for fast-import tends to weigh in at approximately 200 lines of Perl/Python/Ruby code. Most developers have been able to create working importers in just a couple of hours, even though it is their first exposure to fast-import, and sometimes even to Git. This is an ideal situation, given that most conversion tools are throw-away (use once, and never look back).
PARALLEL OPERATIONLike git-push or git-fetch, imports handled by fast-import are safe to run alongside parallel git repack -a -d or git gc invocations, or any other Git operation (including git prune, as loose objects are never used by fast-import).
fast-import does not lock the branch or tag refs it is actively importing. After the import, during its ref update phase, fast-import tests each existing branch ref to verify the update will be a fast-forward update (the commit stored in the ref is contained in the new history of the commit to be written). If the update is not a fast-forward update, fast-import will skip updating that ref and instead prints a warning message. fast-import will always attempt to update all branch refs, and does not stop on the first failure.
TECHNICAL DISCUSSIONfast-import tracks a set of branches in memory. Any branch can be created or modified at any point during the import process by sending a commit command on the input stream. This design allows a frontend program to process an unlimited number of branches simultaneously, generating commits in the order they are available from the source data. It also simplifies the frontend programs considerably.
fast-import does not use or alter the current working directory, or any file within it. (It does however update the current Git repository, as referenced by GIT_DIR.) Therefore an import frontend may use the working directory for its own purposes, such as extracting file revisions from the foreign source. This ignorance of the working directory also allows fast-import to run very quickly, as it does not need to perform any costly file update operations when switching between branches.
INPUT FORMATWith the exception of raw file data (which Git does not interpret) the fast-import input format is text (ASCII) based. This text based format simplifies development and debugging of frontend programs, especially when a higher level language such as Perl, Python or Ruby is being used.
fast-import is very strict about its input. Where we say SP below we mean exactly one space. Likewise LF means one (and only one) linefeed. Supplying additional whitespace characters will cause unexpected results, such as branch names or file names with leading or trailing spaces in their name, or early termination of fast-import when it encounters unexpected input.
Stream CommentsTo aid in debugging frontends fast-import ignores any line that begins with # (ASCII pound/hash) up to and including the line ending LF. A comment line may contain any sequence of bytes that does not contain an LF and therefore may be used to include any detailed debugging information that might be specific to the frontend and useful when inspecting a fast-import data stream.
Date FormatsThe following date formats are supported. A frontend should select the format it will use for this import by passing the format name in the --date-format=<fmt> command line option.
This is the Git native format and is <time> SP <offutc>. It is also fast-import's default format, if --date-format was not specified.
The time of the event is specified by <time> as the number of seconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and is written as an ASCII decimal integer.
The local offset is specified by <offutc> as a positive or negative offset from UTC. For example EST (which is 5 hours behind UTC) would be expressed in <tz> by lq-0500rq while UTC is lq+0000rq. The local offset does not affect <time>; it is used only as an advisement to help formatting routines display the timestamp.
If the local offset is not available in the source material, use lq+0000rq, or the most common local offset. For example many organizations have a CVS repository which has only ever been accessed by users who are located in the same location and timezone. In this case a reasonable offset from UTC could be assumed.
Unlike the rfc2822 format, this format is very strict. Any variation in formatting will cause fast-import to reject the value.
This is the standard email format as described by RFC 2822.
An example value is lqTue Feb 6 11:22:18 2007 -0500rq. The Git parser is accurate, but a little on the lenient side. It is the same parser used by git-am(1) when applying patches received from email.
Some malformed strings may be accepted as valid dates. In some of these cases Git will still be able to obtain the correct date from the malformed string. There are also some types of malformed strings which Git will parse wrong, and yet consider valid. Seriously malformed strings will be rejected.
Unlike the raw format above, the timezone/UTC offset information contained in an RFC 2822 date string is used to adjust the date value to UTC prior to storage. Therefore it is important that this information be as accurate as possible.
If the source material uses RFC 2822 style dates, the frontend should let fast-import handle the parsing and conversion (rather than attempting to do it itself) as the Git parser has been well tested in the wild.
Frontends should prefer the raw format if the source material already uses UNIX-epoch format, can be coaxed to give dates in that format, or its format is easily convertible to it, as there is no ambiguity in parsing.
Always use the current time and timezone. The literal now must always be supplied for <when>.
This is a toy format. The current time and timezone of this system is always copied into the identity string at the time it is being created by fast-import. There is no way to specify a different time or timezone.
This particular format is supplied as its short to implement and may be useful to a process that wants to create a new commit right now, without needing to use a working directory or git-update-index(1).
If separate author and committer commands are used in a commit the timestamps may not match, as the system clock will be polled twice (once for each command). The only way to ensure that both author and committer identity information has the same timestamp is to omit author (thus copying from committer) or to use a date format other than now.
Commandsfast-import accepts several commands to update the current repository and control the current import process. More detailed discussion (with examples) of each command follows later.
- Creates a new branch or updates an existing branch by creating a new commit and updating the branch to point at the newly created commit.
- Creates an annotated tag object from an existing commit or branch. Lightweight tags are not supported by this command, as they are not recommended for recording meaningful points in time.
- Reset an existing branch (or a new branch) to a specific revision. This command must be used to change a branch to a specific revision without making a commit on it.
- Convert raw file data into a blob, for future use in a commit command. This command is optional and is not needed to perform an import.
- Forces fast-import to close the current packfile, generate its unique SHA-1 checksum and index, and start a new packfile. This command is optional and is not needed to perform an import.
- Causes fast-import to echo the entire line to its own standard output. This command is optional and is not needed to perform an import.
commitCreate or update a branch with a new commit, recording one logical change to the project.
'commit' SP <ref> LF mark? ('author' SP <name> SP LT <email> GT SP <when> LF)? 'committer' SP <name> SP LT <email> GT SP <when> LF data ('from' SP <committish> LF)? ('merge' SP <committish> LF)? (filemodify | filedelete | filecopy | filerename | filedeleteall)* LF?
A mark command may optionally appear, requesting fast-import to save a reference to the newly created commit for future use by the frontend (see below for format). It is very common for frontends to mark every commit they create, thereby allowing future branch creation from any imported commit.
The data command following committer must supply the commit message (see below for data command syntax). To import an empty commit message use a 0 length data. Commit messages are free-form and are not interpreted by Git. Currently they must be encoded in UTF-8, as fast-import does not permit other encodings to be specified.
Zero or more filemodify, filedelete, filecopy, filerename and filedeleteall commands may be included to update the contents of the branch prior to creating the commit. These commands may be supplied in any order. However it is recommended that a filedeleteall command precede all filemodify, filecopy and filerename commands in the same commit, as filedeleteall wipes the branch clean (see below).
The LF after the command is optional (it used to be required).
- An author command may optionally appear, if the author information might differ from the committer information. If author is omitted then fast-import will automatically use the committer's information for the author portion of the commit. See below for a description of the fields in author, as they are identical to committer.
The committer command indicates who made this commit, and when they made it.
Here <name> is the person's display name (for example lqCom M Itterrq) and <email> is the person's email address (email@example.com). LT and GT are the literal less-than (\x3c) and greater-than (\x3e) symbols. These are required to delimit the email address from the other fields in the line. Note that <name> is free-form and may contain any sequence of bytes, except LT and LF. It is typically UTF-8 encoded.
The time of the change is specified by <when> using the date format that was selected by the --date-format=<fmt> command line option. See lqDate Formatsrq above for the set of supported formats, and their syntax.
The from command is used to specify the commit to initialize this branch from. This revision will be the first ancestor of the new commit.
Omitting the from command in the first commit of a new branch will cause fast-import to create that commit with no ancestor. This tends to be desired only for the initial commit of a project. Omitting the from command on existing branches is usually desired, as the current commit on that branch is automatically assumed to be the first ancestor of the new commit.
As LF is not valid in a Git refname or SHA-1 expression, no quoting or escaping syntax is supported within <committish>.
Here <committish> is any of the following:
- *The name of an existing branch already in fast-import's internal branch table. If fast-import doesn't know the name, its treated as a SHA-1 expression.
*A mark reference, :<idnum>, where <idnum> is the mark number.
The reason fast-import uses : to denote a mark reference is this character is not legal in a Git branch name. The leading : makes it easy to distinguish between the mark 42 (:42) and the branch 42 (42 or refs/heads/42), or an abbreviated SHA-1 which happened to consist only of base-10 digits.
Marks must be declared (via mark) before they can be used.
- *A complete 40 byte or abbreviated commit SHA-1 in hex.
- *Any valid Git SHA-1 expression that resolves to a commit. See lqSPECIFYING REVISIONSrq in git-rev-parse(1) for details.
.ft C from refs/heads/branch^0 .ft
Includes one additional ancestor commit, and makes the current commit a merge commit. An unlimited number of merge commands per commit are permitted by fast-import, thereby establishing an n-way merge. However Git's other tools never create commits with more than 15 additional ancestors (forming a 16-way merge). For this reason it is suggested that frontends do not use more than 15 merge commands per commit.
Here <committish> is any of the commit specification expressions also accepted by from (see above).
Included in a commit command to add a new file or change the content of an existing file. This command has two different means of specifying the content of the file.
External data format
The data content for the file was already supplied by a prior blob command. The frontend just needs to connect it.
'M' SP <mode> SP <dataref> SP <path> LF
Inline data format
The data content for the file has not been supplied yet. The frontend wants to supply it as part of this modify command.
'M' SP <mode> SP 'inline' SP <path> LF data
- *100644 or 644: A normal (not-executable) file. The majority of files in most projects use this mode. If in doubt, this is what you want.
- *100755 or 755: A normal, but executable, file.
- *120000: A symlink, the content of the file will be the link target.
A <path> string must use UNIX-style directory separators (forward slash /), may contain any byte other than LF, and must not start with double quote (").
If an LF or double quote must be encoded into <path> shell-style quoting should be used, e.g. "path/with\n and \" in it".
The value of <path> must be in canonical form. That is it must not:
- *contain an empty directory component (e.g. foo//bar is invalid),
- *end with a directory separator (e.g. foo/ is invalid),
- *start with a directory separator (e.g. /foo is invalid),
- *contain the special component . or .. (e.g. foo/./bar and foo/../bar are invalid).
- The data content for the file was already supplied by a prior blob command. The frontend just needs to connect it.
Included in a commit command to remove a file or recursively delete an entire directory from the branch. If the file or directory removal makes its parent directory empty, the parent directory will be automatically removed too. This cascades up the tree until the first non-empty directory or the root is reached.
'D' SP <path> LF
Recursively copies an existing file or subdirectory to a different location within the branch. The existing file or directory must exist. If the destination exists it will be completely replaced by the content copied from the source.
'C' SP <path> SP <path> LF
A filecopy command takes effect immediately. Once the source location has been copied to the destination any future commands applied to the source location will not impact the destination of the copy.
Renames an existing file or subdirectory to a different location within the branch. The existing file or directory must exist. If the destination exists it will be replaced by the source directory.
'R' SP <path> SP <path> LF
A filerename command takes effect immediately. Once the source location has been renamed to the destination any future commands applied to the source location will create new files there and not impact the destination of the rename.
Note that a filerename is the same as a filecopy followed by a filedelete of the source location. There is a slight performance advantage to using filerename, but the advantage is so small that it is never worth trying to convert a delete/add pair in source material into a rename for fast-import. This filerename command is provided just to simplify frontends that already have rename information and don't want bother with decomposing it into a filecopy followed by a filedelete.
Included in a commit command to remove all files (and also all directories) from the branch. This command resets the internal branch structure to have no files in it, allowing the frontend to subsequently add all interesting files from scratch.
Issuing a filedeleteall followed by the needed filemodify commands to set the correct content will produce the same results as sending only the needed filemodify and filedelete commands. The filedeleteall approach may however require fast-import to use slightly more memory per active branch (less than 1 MiB for even most large projects); so frontends that can easily obtain only the affected paths for a commit are encouraged to do so.
markArranges for fast-import to save a reference to the current object, allowing the frontend to recall this object at a future point in time, without knowing its SHA-1. Here the current object is the object creation command the mark command appears within. This can be commit, tag, and blob, but commit is the most common usage.
'mark' SP ':' <idnum> LF
tagCreates an annotated tag referring to a specific commit. To create lightweight (non-annotated) tags see the reset command below.
'tag' SP <name> LF 'from' SP <committish> LF 'tagger' SP <name> SP LT <email> GT SP <when> LF data
Tag names are automatically prefixed with refs/tags/ when stored in Git, so importing the CVS branch symbol RELENG-1_0-FINAL would use just RELENG-1_0-FINAL for <name>, and fast-import will write the corresponding ref as refs/tags/RELENG-1_0-FINAL.
The value of <name> must be a valid refname in Git and therefore may contain forward slashes. As LF is not valid in a Git refname, no quoting or escaping syntax is supported here.
The from command is the same as in the commit command; see above for details.
The tagger command uses the same format as committer within commit; again see above for details.
The data command following tagger must supply the annotated tag message (see below for data command syntax). To import an empty tag message use a 0 length data. Tag messages are free-form and are not interpreted by Git. Currently they must be encoded in UTF-8, as fast-import does not permit other encodings to be specified.
Signing annotated tags during import from within fast-import is not supported. Trying to include your own PGP/GPG signature is not recommended, as the frontend does not (easily) have access to the complete set of bytes which normally goes into such a signature. If signing is required, create lightweight tags from within fast-import with reset, then create the annotated versions of those tags offline with the standard git-tag(1) process.
resetCreates (or recreates) the named branch, optionally starting from a specific revision. The reset command allows a frontend to issue a new from command for an existing branch, or to create a new branch from an existing commit without creating a new commit.
'reset' SP <ref> LF ('from' SP <committish> LF)? LF?
The LF after the command is optional (it used to be required).
The reset command can also be used to create lightweight (non-annotated) tags. For example:
reset refs/tags/938 from :938
blobRequests writing one file revision to the packfile. The revision is not connected to any commit; this connection must be formed in a subsequent commit command by referencing the blob through an assigned mark.
'blob' LF mark? data
dataSupplies raw data (for use as blob/file content, commit messages, or annotated tag messages) to fast-import. Data can be supplied using an exact byte count or delimited with a terminating line. Real frontends intended for production-quality conversions should always use the exact byte count format, as it is more robust and performs better. The delimited format is intended primarily for testing fast-import.
Comment lines appearing within the <raw> part of data commands are always taken to be part of the body of the data and are therefore never ignored by fast-import. This makes it safe to import any file/message content whose lines might start with #.
Exact byte count format
The frontend must specify the number of bytes of data.
'data' SP <count> LF <raw> LF?
The LF after <raw> is optional (it used to be required) but recommended. Always including it makes debugging a fast-import stream easier as the next command always starts in column 0 of the next line, even if <raw> did not end with an LF.
A delimiter string is used to mark the end of the data. fast-import will compute the length by searching for the delimiter. This format is primarily useful for testing and is not recommended for real data.
'data' SP '<<' <delim> LF <raw> LF <delim> LF LF?
The LF after <delim> LF is optional (it used to be required).
checkpointForces fast-import to close the current packfile, start a new one, and to save out all current branch refs, tags and marks.
'checkpoint' LF LF?
As a checkpoint can require a significant amount of CPU time and disk IO (to compute the overall pack SHA-1 checksum, generate the corresponding index file, and update the refs) it can easily take several minutes for a single checkpoint command to complete.
Frontends may choose to issue checkpoints during extremely large and long running imports, or when they need to allow another Git process access to a branch. However given that a 30 GiB Subversion repository can be loaded into Git through fast-import in about 3 hours, explicit checkpointing may not be necessary.
progressCauses fast-import to print the entire progress line unmodified to its standard output channel (file descriptor 1) when the command is processed from the input stream. The command otherwise has no impact on the current import, or on any of fast-import's internal state.
'progress' SP <any> LF LF?
frontend | git-fast-import | sed 's/^progress //'
TIPS AND TRICKSThe following tips and tricks have been collected from various users of fast-import, and are offered here as suggestions.
Use One Mark Per CommitWhen doing a repository conversion, use a unique mark per commit (mark :<n>) and supply the --export-marks option on the command line. fast-import will dump a file which lists every mark and the Git object SHA-1 that corresponds to it. If the frontend can tie the marks back to the source repository, it is easy to verify the accuracy and completeness of the import by comparing each Git commit to the corresponding source revision.
Freely Skip Around BranchesDon't bother trying to optimize the frontend to stick to one branch at a time during an import. Although doing so might be slightly faster for fast-import, it tends to increase the complexity of the frontend code considerably.
The branch LRU builtin to fast-import tends to behave very well, and the cost of activating an inactive branch is so low that bouncing around between branches has virtually no impact on import performance.
Handling RenamesWhen importing a renamed file or directory, simply delete the old name(s) and modify the new name(s) during the corresponding commit. Git performs rename detection after-the-fact, rather than explicitly during a commit.
Use Tag Fixup BranchesSome other SCM systems let the user create a tag from multiple files which are not from the same commit/changeset. Or to create tags which are a subset of the files available in the repository.
Importing these tags as-is in Git is impossible without making at least one commit which lqfixes uprq the files to match the content of the tag. Use fast-import's reset command to reset a dummy branch outside of your normal branch space to the base commit for the tag, then commit one or more file fixup commits, and finally tag the dummy branch.
For example since all normal branches are stored under refs/heads/ name the tag fixup branch TAG_FIXUP. This way it is impossible for the fixup branch used by the importer to have namespace conflicts with real branches imported from the source (the name TAG_FIXUP is not refs/heads/TAG_FIXUP).
When committing fixups, consider using merge to connect the commit(s) which are supplying file revisions to the fixup branch. Doing so will allow tools such as git-blame(1) to track through the real commit history and properly annotate the source files.
Import Now, Repack LaterAs soon as fast-import completes the Git repository is completely valid and ready for use. Typically this takes only a very short time, even for considerably large projects (100,000+ commits).
However repacking the repository is necessary to improve data locality and access performance. It can also take hours on extremely large projects (especially if -f and a large --window parameter is used). Since repacking is safe to run alongside readers and writers, run the repack in the background and let it finish when it finishes. There is no reason to wait to explore your new Git project!
If you choose to wait for the repack, don't try to run benchmarks or performance tests until repacking is completed. fast-import outputs suboptimal packfiles that are simply never seen in real use situations.
Repacking Historical DataIf you are repacking very old imported data (e.g. older than the last year), consider expending some extra CPU time and supplying --window=50 (or higher) when you run git-repack(1). This will take longer, but will also produce a smaller packfile. You only need to expend the effort once, and everyone using your project will benefit from the smaller repository.
Include Some Progress MessagesEvery once in a while have your frontend emit a progress message to fast-import. The contents of the messages are entirely free-form, so one suggestion would be to output the current month and year each time the current commit date moves into the next month. Your users will feel better knowing how much of the data stream has been processed.
PACKFILE OPTIMIZATIONWhen packing a blob fast-import always attempts to deltify against the last blob written. Unless specifically arranged for by the frontend, this will probably not be a prior version of the same file, so the generated delta will not be the smallest possible. The resulting packfile will be compressed, but will not be optimal.
Frontends which have efficient access to all revisions of a single file (for example reading an RCS/CVS ,v file) can choose to supply all revisions of that file as a sequence of consecutive blob commands. This allows fast-import to deltify the different file revisions against each other, saving space in the final packfile. Marks can be used to later identify individual file revisions during a sequence of commit commands.
The packfile(s) created by fast-import do not encourage good disk access patterns. This is caused by fast-import writing the data in the order it is received on standard input, while Git typically organizes data within packfiles to make the most recent (current tip) data appear before historical data. Git also clusters commits together, speeding up revision traversal through better cache locality.
For this reason it is strongly recommended that users repack the repository with git repack -a -d after fast-import completes, allowing Git to reorganize the packfiles for faster data access. If blob deltas are suboptimal (see above) then also adding the -f option to force recomputation of all deltas can significantly reduce the final packfile size (30-50% smaller can be quite typical).
MEMORY UTILIZATIONThere are a number of factors which affect how much memory fast-import requires to perform an import. Like critical sections of core Git, fast-import uses its own memory allocators to amortize any overheads associated with malloc. In practice fast-import tends to amortize any malloc overheads to 0, due to its use of large block allocations.
per objectfast-import maintains an in-memory structure for every object written in this execution. On a 32 bit system the structure is 32 bytes, on a 64 bit system the structure is 40 bytes (due to the larger pointer sizes). Objects in the table are not deallocated until fast-import terminates. Importing 2 million objects on a 32 bit system will require approximately 64 MiB of memory.
The object table is actually a hashtable keyed on the object name (the unique SHA-1). This storage configuration allows fast-import to reuse an existing or already written object and avoid writing duplicates to the output packfile. Duplicate blobs are surprisingly common in an import, typically due to branch merges in the source.
per markMarks are stored in a sparse array, using 1 pointer (4 bytes or 8 bytes, depending on pointer size) per mark. Although the array is sparse, frontends are still strongly encouraged to use marks between 1 and n, where n is the total number of marks required for this import.
per branchBranches are classified as active and inactive. The memory usage of the two classes is significantly different.
Inactive branches are stored in a structure which uses 96 or 120 bytes (32 bit or 64 bit systems, respectively), plus the length of the branch name (typically under 200 bytes), per branch. fast-import will easily handle as many as 10,000 inactive branches in under 2 MiB of memory.
Active branches have the same overhead as inactive branches, but also contain copies of every tree that has been recently modified on that branch. If subtree include has not been modified since the branch became active, its contents will not be loaded into memory, but if subtree src has been modified by a commit since the branch became active, then its contents will be loaded in memory.
As active branches store metadata about the files contained on that branch, their in-memory storage size can grow to a considerable size (see below).
fast-import automatically moves active branches to inactive status based on a simple least-recently-used algorithm. The LRU chain is updated on each commit command. The maximum number of active branches can be increased or decreased on the command line with --active-branches=.
per active treeTrees (aka directories) use just 12 bytes of memory on top of the memory required for their entries (see lqper active filerq below). The cost of a tree is virtually 0, as its overhead amortizes out over the individual file entries.
per active file entryFiles (and pointers to subtrees) within active trees require 52 or 64 bytes (32/64 bit platforms) per entry. To conserve space, file and tree names are pooled in a common string table, allowing the filename lqMakefilerq to use just 16 bytes (after including the string header overhead) no matter how many times it occurs within the project.
The active branch LRU, when coupled with the filename string pool and lazy loading of subtrees, allows fast-import to efficiently import projects with 2,000+ branches and 45,114+ files in a very limited memory footprint (less than 2.7 MiB per active branch).
AUTHORWritten by Shawn O. Pearce <firstname.lastname@example.org>.
DOCUMENTATIONDocumentation by Shawn O. Pearce <email@example.com>.
GITPart of the git(7) suite
- DEVELOPMENT COST
- PARALLEL OPERATION
- TECHNICAL DISCUSSION
- INPUT FORMAT
- TIPS AND TRICKS
- PACKFILE OPTIMIZATION
- MEMORY UTILIZATION