On Wed, Mar 17, 2021 at 10:12 PM Matheus Tavares
[off-list ref] 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);