| Message ID | 0592740ec14c4ab72b8d46a7fecf1c66e7a497fd.1616015337.git.matheus.bernardino@usp.br (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | Parallel Checkout (part 2) | expand |
On Wed, Mar 17, 2021 at 10:12 PM Matheus Tavares <matheus.bernardino@usp.br> wrote: > +For the purposes of discussion here, the current sequential > +implementation of Step 3 has 3 layers: You refer to these layers as "steps" below, so you might want to use "sub-steps" instead of "layers". > +* Step 3a: `unpack-trees.c:check_updates()` contains a series of > + sequential loops iterating over the `cache_entry`'s array. The main > + loop in this function calls the next layer for each of the Not sure what "layer" means here. Does it mean Step 3b below? In this case I would suggest using "Step 3b" instead of "layer". > + to-be-updated entries. > + > +* Step 3b: `entry.c:checkout_entry()` examines the existing working tree > + for file conflicts, collisions, and unsaved changes. It removes files > + and create leading directories as necessary. It calls the next layer s/create/creates/ I guess the "next layer" is Step 3c below. > + for each entry to be written. > + > +* Step 3c: `entry.c:write_entry()` loads the blob into memory, smudges > + it if necessary, creates the file in the working tree, writes the > + smudged contents, calls `fstat()` or `lstat()`, and updates the > + associated `cache_entry` struct with the stat information gathered. > + > +It wouldn't be safe to perform Step 3b in parallel, as there could be > +race conditions between file creations and removals. Instead, the > +parallel checkout framework lets the sequential code handle Step 3b, > +and use parallel workers to replace the sequential > +`entry.c:write_entry()` calls from Step 3c. Ok. > +Rejected Multi-Threaded Solution > +-------------------------------- > + > +The most "straightforward" implementation would be to spread the set of > +to-be-updated cache entries across multiple threads. But due to the > +thread-unsafe functions in the ODB code, we would have to use locks to > +coordinate the parallel operation. An early prototype of this solution > +showed that the multi-threaded checkout would bring performance > +improvements over the sequential code, but there was still too much lock > +contention. A `perf` profiling indicated that around 20% of the runtime > +during a local Linux clone (on an SSD) was spent in locking functions. > +For this reason this approach was rejected in favor of using multiple > +child processes, which led to a better performance. Nice explanation. > +Multi-Process Solution > +---------------------- > + > +Parallel checkout alters the aforementioned Step 3 to use multiple > +`checkout--helper` background processes to distribute the work. The > +long-running worker processes are controlled by the foreground Git > +command using the existing run-command API. > + > +Overview > +~~~~~~~~ > + > +Step 3b is only slightly altered; for each entry to be checked out, the > +main process: Maybe: s/main process:/main process performs the following steps:/ If you apply this suggestion, you may also want the following below: s/M1: Checks/M1: Check/ s/and decides/and decide/ s/M2: Creates/M2: Create/ ... > +* M1: Checks whether there is any untracked or unclean file in the > + working tree which would be overwritten by this entry, and decides > + whether to proceed (removing the file(s)) or not. > + > +* M2: Creates the leading directories. > + > +* M3: Loads the conversion attributes for the entry's path. > + > +* M4: Checks, based on the entry's type and conversion attributes, > + whether the entry is eligible for parallel checkout (more on this > + later). If it is eligible, enqueues the entry and the loaded > + attributes to later write the entry in parallel. If not, writes the > + entry right away, using the default sequential code. > + > +Note: we save the conversion attributes associated with each entry > +because the workers don't have access to the main process' index state, > +so they can't load the attributes by themselves (and the attributes are > +needed to properly smudge the entry). Additionally, this has a positive > +impact on performance as (1) we don't need to load the attributes twice > +and (2) the attributes machinery is optimized to handle paths in > +sequential order. Nice! > +After all entries have passed through the above steps, the main process > +checks if the number of enqueued entries is sufficient to spread among > +the workers. If not, it just writes them sequentially. Otherwise, it > +spawns the workers and distributes the queued entries uniformly in > +continuous chunks. This aims to minimize the chances of two workers > +writing to the same directory simultaneously, which could increase lock > +contention in the kernel. > + > +Then, for each assigned item, each worker: > + > +* W1: Checks if there is any non-directory file in the leading part of > + the entry's path or if there already exists a file at the entry' path. > + If so, mark the entry with `PC_ITEM_COLLIDED` and skip it (more on > + this later). > + > +* W2: Creates the file (with O_CREAT and O_EXCL). > + > +* W3: Loads the blob into memory (inflating and delta reconstructing > + it). > + > +* W4: Filters the blob. Not sure what "Filters" means here. Is this related to the smudge filter? > +* W5: Writes the result to the file descriptor opened at W2. > + > +* W6: Calls `fstat()` or lstat()` on the just-written path, and sends > + the result back to the main process, together with the end status of > + the operation and the item's identification number. > + > +Note that steps W3 to W5 might actually be performed together, using the > +streaming interface. Not sure what "performed together" means here. Does it mean by a single function or set of functions? > +Also note that the workers *never* remove any files. As mentioned Maybe: s/any files/any file/ > +earlier, it is the responsibility of the main process to remove any > +files that block the checkout operation (or abort it). This is crucial Maybe: s/files/file/ and s/block/blocks/ and s/abort/aborts/ > +to avoid race conditions and also to properly detect path collisions at > +Step W1. > + > +After the workers finish writing the items and sending back the required > +information, the main process handles the results in two steps: > + > +- First, it updates the in-memory index with the `lstat()` information > + sent by the workers. (This must be done first as this information > + might me required in the following step.) > + > +- Then it writes the items which collided on disk (i.e. items marked > + with `PC_ITEM_COLLIDED`). More on this below. > + > +Path Collisions > +--------------- > + > +Path collisions happen when two different paths correspond to the same > +entry in the file system. E.g. the paths 'a' and 'A' would collide in a > +case-insensitive file system. > + > +The sequential checkout deals with collisions in the same way that it > +deals with files that were already present in the working tree before > +checkout. Basically, it checks if the path that it wants to write > +already exists on disk, makes sure the existing file doesn't have > +unsaved data, and then overwrite it. (To be more pedantic: it deletes s/overwrite/overwrites/ > +the existing file and creates the new one.) So, if there are multiple > +colliding files to be checked out, the sequential code will write each > +one of them but only the last will actually survive on disk. > + > +Parallel checkout aims to reproduce the same behavior. However, we > +cannot let the workers racily write to the same file on disk. Instead, > +the workers detect when the entry that they want to check out would > +collide with an existing file, and mark it with `PC_ITEM_COLLIDED`. > +Later, the main process can sequentially feed these entries back to > +`checkout_entry()` without the risk of race conditions. On clone, this > +also has the effect of marking the colliding entries to later emit a > +warning for the user, like the classic sequential checkout does. > + > +The workers are able to detect both collisions among the entries being > +concurrently written and collisions among parallel-eligible and > +ineligible entries. The general idea for collision detection is quite > +straightforward: for each parallel-eligible entry, the main process must > +remove all files that prevent this entry from being written (before > +enqueueing it). This includes any non-directory file in the leading path > +of the entry. Later, when a worker gets assigned the entry, it looks > +again for the non-directories files and for an already existent file at Maybe: s/existent/existing/ > +the entry's path. If any of these checks finds something, the worker > +knows that there was a path collision. > + > +Because parallel checkout can distinguish path collisions from the case > +where the file was already present in the working tree before checkout, > +we could alternatively choose to skip the checkout of colliding entries. > +However, each entry that doesn't get written would have NULL `lstat()` > +fields on the index. This could cause performance penalties for > +subsequent commands that need to refresh the index, as they would have > +to go to the file system to see if the entry is dirty. Thus, if we have > +N entries in a colliding group and we decide to write and `lstat()` only > +one of them, every subsequent `git-status` will have to read, convert, > +and hash the written file N - 1 times. By checking out all colliding > +entries (like the sequential code does), we only pay the overhead once, > +during checkout. > + > +Eligible Entries for Parallel Checkout > +-------------------------------------- > + > +As previously mentioned, not all entries passed to `checkout_entry()` > +will be considered eligible for parallel checkout. More specifically, we > +exclude: > + > +- Symbolic links; to avoid race conditions that, in combination with > + path collisions, could cause workers to write files at the wrong > + place. For example, if we were to concurrently check out a symlink > + 'a' -> 'b' and a regular file 'A/f' in a case-insensitive file system, > + we could potentially end up writing the file 'A/f' at 'a/f', due to a > + race condition. > + > +- Regular files that require external filters (either "one shot" filters > + or long-running process filters). These filters are black-boxes to Git > + and may have their own internal locking or non-concurrent assumptions. > + So it might not be safe to run multiple instances in parallel. > ++ > +Besides, long-running filters may use the delayed checkout feature to > +postpone the return of some filtered blobs. The delayed checkout queue > +and the parallel checkout queue are not compatible and should remain > +separated. Are files that require some other internal filters eligible though? > +Ineligible entries are checked out by the classic sequential codepath > +*before* spawning workers. > + > +Note: submodules's files are also eligible for parallel checkout (as > +long as they don't fall into the two excluding categories mentioned > +above). But since each submodule is checked out in its own child > +process, we don't mix the superproject's and the submodules' files in > +the same parallel checkout process or queue. Ok. > +The API > +------- > + > +The parallel checkout API was designed with the goal to minimize changes > +to the current users of the checkout machinery. This means that they > +don't have to call a different function for sequential or parallel > +checkout. As already mentioned, `checkout_entry()` will automatically > +insert the given entry in the parallel checkout queue when this feature > +is enabled and the entry is eligible; otherwise, it will just write the > +entry right away, using the sequential code. In general, callers of the > +parallel checkout API should look similar to this: > + > +---------------------------------------------- > +int pc_workers, pc_threshold, err = 0; > +struct checkout state; > + > +get_parallel_checkout_configs(&pc_workers, &pc_threshold); > + > +/* > + * This check is not strictly required, but it > + * should save some time in sequential mode. > + */ It might be nice if this comment was also in front of the real code. > +if (pc_workers > 1) > + init_parallel_checkout(); > + > +for (each cache_entry ce to-be-updated) > + err |= checkout_entry(ce, &state, NULL, NULL); > + > +err |= run_parallel_checkout(&state, pc_workers, pc_threshold, NULL, NULL);
diff --git a/Documentation/Makefile b/Documentation/Makefile index 81d1bf7a04..af236927c9 100644 --- a/Documentation/Makefile +++ b/Documentation/Makefile @@ -90,6 +90,7 @@ TECH_DOCS += technical/multi-pack-index TECH_DOCS += technical/pack-format TECH_DOCS += technical/pack-heuristics TECH_DOCS += technical/pack-protocol +TECH_DOCS += technical/parallel-checkout TECH_DOCS += technical/partial-clone TECH_DOCS += technical/protocol-capabilities TECH_DOCS += technical/protocol-common diff --git a/Documentation/technical/parallel-checkout.txt b/Documentation/technical/parallel-checkout.txt new file mode 100644 index 0000000000..c7f6570f1c --- /dev/null +++ b/Documentation/technical/parallel-checkout.txt @@ -0,0 +1,262 @@ +Parallel Checkout Design Notes +============================== + +The "Parallel Checkout" feature attempts to use multiple processes to +parallelize the work of uncompressing, smudging, and writing blobs +during a checkout operation. It can be used by all checkout-related +commands, such as `clone`, `checkout`, `reset`, `sparse-checkout`, and +others. + +These commands share the following basic structure: + +* Step 1: Read the current index file into memory. + +* Step 2: Modify the in-memory index based upon the command, and + temporarily mark all cache entries that need to be updated. + +* Step 3: Populate the working tree to match the new candidate index. + This includes iterating over all of the to-be-updated cache entries + and delete, create, or overwrite the associated files in the working + tree. + +* Step 4: Write the new index to disk. + +Step 3 is the focus of the "parallel checkout" effort described here. +It dominates the execution time for most of the above command types. + +Sequential Implementation +------------------------- + +For the purposes of discussion here, the current sequential +implementation of Step 3 has 3 layers: + +* Step 3a: `unpack-trees.c:check_updates()` contains a series of + sequential loops iterating over the `cache_entry`'s array. The main + loop in this function calls the next layer for each of the + to-be-updated entries. + +* Step 3b: `entry.c:checkout_entry()` examines the existing working tree + for file conflicts, collisions, and unsaved changes. It removes files + and create leading directories as necessary. It calls the next layer + for each entry to be written. + +* Step 3c: `entry.c:write_entry()` loads the blob into memory, smudges + it if necessary, creates the file in the working tree, writes the + smudged contents, calls `fstat()` or `lstat()`, and updates the + associated `cache_entry` struct with the stat information gathered. + +It wouldn't be safe to perform Step 3b in parallel, as there could be +race conditions between file creations and removals. Instead, the +parallel checkout framework lets the sequential code handle Step 3b, +and use parallel workers to replace the sequential +`entry.c:write_entry()` calls from Step 3c. + +Rejected Multi-Threaded Solution +-------------------------------- + +The most "straightforward" implementation would be to spread the set of +to-be-updated cache entries across multiple threads. But due to the +thread-unsafe functions in the ODB code, we would have to use locks to +coordinate the parallel operation. An early prototype of this solution +showed that the multi-threaded checkout would bring performance +improvements over the sequential code, but there was still too much lock +contention. A `perf` profiling indicated that around 20% of the runtime +during a local Linux clone (on an SSD) was spent in locking functions. +For this reason this approach was rejected in favor of using multiple +child processes, which led to a better performance. + +Multi-Process Solution +---------------------- + +Parallel checkout alters the aforementioned Step 3 to use multiple +`checkout--helper` background processes to distribute the work. The +long-running worker processes are controlled by the foreground Git +command using the existing run-command API. + +Overview +~~~~~~~~ + +Step 3b is only slightly altered; for each entry to be checked out, the +main process: + +* M1: Checks whether there is any untracked or unclean file in the + working tree which would be overwritten by this entry, and decides + whether to proceed (removing the file(s)) or not. + +* M2: Creates the leading directories. + +* M3: Loads the conversion attributes for the entry's path. + +* M4: Checks, based on the entry's type and conversion attributes, + whether the entry is eligible for parallel checkout (more on this + later). If it is eligible, enqueues the entry and the loaded + attributes to later write the entry in parallel. If not, writes the + entry right away, using the default sequential code. + +Note: we save the conversion attributes associated with each entry +because the workers don't have access to the main process' index state, +so they can't load the attributes by themselves (and the attributes are +needed to properly smudge the entry). Additionally, this has a positive +impact on performance as (1) we don't need to load the attributes twice +and (2) the attributes machinery is optimized to handle paths in +sequential order. + +After all entries have passed through the above steps, the main process +checks if the number of enqueued entries is sufficient to spread among +the workers. If not, it just writes them sequentially. Otherwise, it +spawns the workers and distributes the queued entries uniformly in +continuous chunks. This aims to minimize the chances of two workers +writing to the same directory simultaneously, which could increase lock +contention in the kernel. + +Then, for each assigned item, each worker: + +* W1: Checks if there is any non-directory file in the leading part of + the entry's path or if there already exists a file at the entry' path. + If so, mark the entry with `PC_ITEM_COLLIDED` and skip it (more on + this later). + +* W2: Creates the file (with O_CREAT and O_EXCL). + +* W3: Loads the blob into memory (inflating and delta reconstructing + it). + +* W4: Filters the blob. + +* W5: Writes the result to the file descriptor opened at W2. + +* W6: Calls `fstat()` or lstat()` on the just-written path, and sends + the result back to the main process, together with the end status of + the operation and the item's identification number. + +Note that steps W3 to W5 might actually be performed together, using the +streaming interface. + +Also note that the workers *never* remove any files. As mentioned +earlier, it is the responsibility of the main process to remove any +files that block the checkout operation (or abort it). This is crucial +to avoid race conditions and also to properly detect path collisions at +Step W1. + +After the workers finish writing the items and sending back the required +information, the main process handles the results in two steps: + +- First, it updates the in-memory index with the `lstat()` information + sent by the workers. (This must be done first as this information + might me required in the following step.) + +- Then it writes the items which collided on disk (i.e. items marked + with `PC_ITEM_COLLIDED`). More on this below. + +Path Collisions +--------------- + +Path collisions happen when two different paths correspond to the same +entry in the file system. E.g. the paths 'a' and 'A' would collide in a +case-insensitive file system. + +The sequential checkout deals with collisions in the same way that it +deals with files that were already present in the working tree before +checkout. Basically, it checks if the path that it wants to write +already exists on disk, makes sure the existing file doesn't have +unsaved data, and then overwrite it. (To be more pedantic: it deletes +the existing file and creates the new one.) So, if there are multiple +colliding files to be checked out, the sequential code will write each +one of them but only the last will actually survive on disk. + +Parallel checkout aims to reproduce the same behavior. However, we +cannot let the workers racily write to the same file on disk. Instead, +the workers detect when the entry that they want to check out would +collide with an existing file, and mark it with `PC_ITEM_COLLIDED`. +Later, the main process can sequentially feed these entries back to +`checkout_entry()` without the risk of race conditions. On clone, this +also has the effect of marking the colliding entries to later emit a +warning for the user, like the classic sequential checkout does. + +The workers are able to detect both collisions among the entries being +concurrently written and collisions among parallel-eligible and +ineligible entries. The general idea for collision detection is quite +straightforward: for each parallel-eligible entry, the main process must +remove all files that prevent this entry from being written (before +enqueueing it). This includes any non-directory file in the leading path +of the entry. Later, when a worker gets assigned the entry, it looks +again for the non-directories files and for an already existent file at +the entry's path. If any of these checks finds something, the worker +knows that there was a path collision. + +Because parallel checkout can distinguish path collisions from the case +where the file was already present in the working tree before checkout, +we could alternatively choose to skip the checkout of colliding entries. +However, each entry that doesn't get written would have NULL `lstat()` +fields on the index. This could cause performance penalties for +subsequent commands that need to refresh the index, as they would have +to go to the file system to see if the entry is dirty. Thus, if we have +N entries in a colliding group and we decide to write and `lstat()` only +one of them, every subsequent `git-status` will have to read, convert, +and hash the written file N - 1 times. By checking out all colliding +entries (like the sequential code does), we only pay the overhead once, +during checkout. + +Eligible Entries for Parallel Checkout +-------------------------------------- + +As previously mentioned, not all entries passed to `checkout_entry()` +will be considered eligible for parallel checkout. More specifically, we +exclude: + +- Symbolic links; to avoid race conditions that, in combination with + path collisions, could cause workers to write files at the wrong + place. For example, if we were to concurrently check out a symlink + 'a' -> 'b' and a regular file 'A/f' in a case-insensitive file system, + we could potentially end up writing the file 'A/f' at 'a/f', due to a + race condition. + +- Regular files that require external filters (either "one shot" filters + or long-running process filters). These filters are black-boxes to Git + and may have their own internal locking or non-concurrent assumptions. + So it might not be safe to run multiple instances in parallel. ++ +Besides, long-running filters may use the delayed checkout feature to +postpone the return of some filtered blobs. The delayed checkout queue +and the parallel checkout queue are not compatible and should remain +separated. + +Ineligible entries are checked out by the classic sequential codepath +*before* spawning workers. + +Note: submodules's files are also eligible for parallel checkout (as +long as they don't fall into the two excluding categories mentioned +above). But since each submodule is checked out in its own child +process, we don't mix the superproject's and the submodules' files in +the same parallel checkout process or queue. + +The API +------- + +The parallel checkout API was designed with the goal to minimize changes +to the current users of the checkout machinery. This means that they +don't have to call a different function for sequential or parallel +checkout. As already mentioned, `checkout_entry()` will automatically +insert the given entry in the parallel checkout queue when this feature +is enabled and the entry is eligible; otherwise, it will just write the +entry right away, using the sequential code. In general, callers of the +parallel checkout API should look similar to this: + +---------------------------------------------- +int pc_workers, pc_threshold, err = 0; +struct checkout state; + +get_parallel_checkout_configs(&pc_workers, &pc_threshold); + +/* + * This check is not strictly required, but it + * should save some time in sequential mode. + */ +if (pc_workers > 1) + init_parallel_checkout(); + +for (each cache_entry ce to-be-updated) + err |= checkout_entry(ce, &state, NULL, NULL); + +err |= run_parallel_checkout(&state, pc_workers, pc_threshold, NULL, NULL); +----------------------------------------------
Co-authored-by: Jeff Hostetler <jeffhost@microsoft.com> Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br> --- Documentation/Makefile | 1 + Documentation/technical/parallel-checkout.txt | 262 ++++++++++++++++++ 2 files changed, 263 insertions(+) create mode 100644 Documentation/technical/parallel-checkout.txt