Unpack trees with cache tree gvfs #23
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Performance measurements are listed right now as a flat list, which is fine when we measure big blocks. But when we start adding more and more measurements, some of them could be just part of a bigger measurement and a flat list gives a wrong impression that they are executed at the same level instead of nested. Add trace_performance_enter() and trace_performance_leave() to allow indent these nested measurements. For now it does not help much because the only nested thing is (lazy) name hash initialization (e.g. called in diff-index from "git status"). This will help more because I'm going to add some more tracing that's actually nested. Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]> Signed-off-by: Junio C Hamano <[email protected]>
We're going to optimize unpack_trees() a bit in the following
patches. Let's add some tracing to measure how long it takes before
and after. This is the baseline ("git checkout -" on webkit.git, 275k
files on worktree)
performance: 0.056651714 s: read cache .git/index
performance: 0.183101080 s: preload index
performance: 0.008584433 s: refresh index
performance: 0.633767589 s: traverse_trees
performance: 0.340265448 s: check_updates
performance: 0.381884638 s: cache_tree_update
performance: 1.401562947 s: unpack_trees
performance: 0.338687914 s: write index, changed mask = 2e
performance: 0.411927922 s: traverse_trees
performance: 0.000023335 s: check_updates
performance: 0.423697246 s: unpack_trees
performance: 0.423708360 s: diff-index
performance: 2.559524127 s: git command: git checkout -
Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
In order to merge one or many trees with the index, unpack-trees code
walks multiple trees in parallel with the index and performs n-way
merge. If we find out at start of a directory that all trees are the
same (by comparing OID) and cache-tree happens to be available for
that directory as well, we could avoid walking the trees because we
already know what these trees contain: it's flattened in what's called
"the index".
The upside is of course a lot less I/O since we can potentially skip
lots of trees (think subtrees). We also save CPU because we don't have
to inflate and apply the deltas. The downside is of course more
fragile code since the logic in some functions are now duplicated
elsewhere.
"checkout -" with this patch on webkit.git (275k files):
baseline new
--------------------------------------------------------------------
0.056651714 0.080394752 s: read cache .git/index
0.183101080 0.216010838 s: preload index
0.008584433 0.008534301 s: refresh index
0.633767589 0.251992198 s: traverse_trees
0.340265448 0.377031383 s: check_updates
0.381884638 0.372768105 s: cache_tree_update
1.401562947 1.045887251 s: unpack_trees
0.338687914 0.314983512 s: write index, changed mask = 2e
0.411927922 0.062572653 s: traverse_trees
0.000023335 0.000022544 s: check_updates
0.423697246 0.073795585 s: unpack_trees
0.423708360 0.073807557 s: diff-index
2.559524127 1.938191592 s: git command: git checkout -
Another measurement from Ben's running "git checkout" with over 500k
trees (on the whole series):
baseline new
----------------------------------------------------------------------
0.535510167 0.556558733 s: read cache .git/index
0.3057373 0.3147105 s: initialize name hash
0.0184082 0.023558433 s: preload index
0.086910967 0.089085967 s: refresh index
7.889590767 2.191554433 s: unpack trees
0.120760833 0.131941267 s: update worktree after a merge
2.2583504 2.572663167 s: repair cache-tree
0.8916137 0.959495233 s: write index, changed mask = 28
3.405199233 0.2710663 s: unpack trees
0.000999667 0.0021554 s: update worktree after a merge
3.4063306 0.273318333 s: diff-index
16.9524923 9.462943133 s: git command: git.exe checkout
This command calls unpack_trees() twice, the first time on 2way merge
and the second 1way merge. In both times, "unpack trees" time is
reduced to one third. Overall time reduction is not that impressive of
course because index operations take a big chunk. And there's that
repair cache-tree line.
PS. A note about cache-tree invalidation and the use of it in this
code.
We do invalidate cache-tree in _source_ index when we add new entries
to the (temporary) "result" index. But we also use the cache-tree from
source index in this optimization. Does this mean we end up having no
cache-tree in the source index to activate this optimization?
The answer is twisted: the order of finding a good cache-tree and
invalidating it matters. In this case we check for a good cache-tree
first in all_trees_same_as_cache_tree(), then we start to merge things
and potentially invalidate that same cache-tree in the process. Since
cache-tree invalidation happens after the optimization kicks in, we're
still good. But we may lose that cache-tree at the very first
call_unpack_fn() call in traverse_by_cache_tree().
Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
This is a micro optimization that probably only shines on repos with deep directory structure. Instead of allocating and freeing a new cache_entry in every iteration, we reuse the last one and only update the parts that are new each iteration. Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]> Signed-off-by: Junio C Hamano <[email protected]>
We do n-way merge by walking the source index and n trees at the same time and add merge results to a new temporary index called o->result. The merge result for any given path could be either - keep_entry(): same old index entry in o->src_index is reused - merged_entry(): either a new entry is added, or an existing one updated - deleted_entry(): one entry from o->src_index is removed For some reason [1] we keep making sure that the source index's cache-tree is still valid if used by o->result: for all those merged/deleted entries, we invalidate the same path in o->src_index, so only cache-trees covering the "keep_entry" parts remain good. Because of this, the cache-tree from o->src_index can be perfectly reused in o->result. And in fact we already rely on this logic to reuse untracked cache in edf3b90 (unpack-trees: preserve index extensions - 2017-05-08). Move the cache-tree to o->result before doing cache_tree_update() to reduce hashing cost. Since cache_tree_update() has risen up as one of the most expensive parts in unpack_trees() after the last few patches. This does help reduce unpack_trees() time significantly (on webkit.git): before after -------------------------------------------------------------------- 0.080394752 0.051258167 s: read cache .git/index 0.216010838 0.212106298 s: preload index 0.008534301 0.280521764 s: refresh index 0.251992198 0.218160442 s: traverse_trees 0.377031383 0.374948191 s: check_updates 0.372768105 0.037040114 s: cache_tree_update 1.045887251 0.672031609 s: unpack_trees 0.314983512 0.317456290 s: write index, changed mask = 2e 0.062572653 0.038382654 s: traverse_trees 0.000022544 0.000042731 s: check_updates 0.073795585 0.050930053 s: unpack_trees 0.073807557 0.051099735 s: diff-index 1.938191592 1.614241153 s: git command: git checkout - [1] I'm pretty sure the reason is an oversight in 34110cd (Make 'unpack_trees()' have a separate source and destination index - 2008-03-06). That patch aims to _not_ update the source index at all. The invalidation should have been done on o->result in that patch. But then there was no cache-tree on o->result even then so it's pointless to do so. Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]> Signed-off-by: Junio C Hamano <[email protected]>
Any changes to the output index should be (confusingly) marked in the source index with invalidate_ce_path(). This is used to make sure we still have valid untracked cache and cache-tree extensions in the end. We do a pretty good job of invalidating except in two places. verify_clean_subdirectory() is part of verify_absent() and verify_absent_sparse(). The former is usually called by merged_entry() or directly in threeway_merge(). The latter is obviously used by sparse checkout. In these three call sites, only merged_entry() follows up with invalidate_ce_path(). The other two don't, but they should not trigger this ce removal because this is about D/F conflicts [1]. But let's be safe and invalidate_ce_path() here as well. The second place is keep_entry() which is also used by threeway_merge() to keep higher stage entries. In order to reuse cache-tree we need to invalidate these paths as well. It's not a problem in the past because whenever a higher stage entry is present, cache-tree will not be created [2]. Now we salvage cache-tree even when higher stage entries are present, we need more invalidation. [1] c819353 (Fix switching to a branch with D/F when current branch has file D. - 2007-03-15) [2] This is probably too strict. We should be able to create and save cache-tree for the directories that do not have conflict entries in cache_tree_update(). And this becomes more important when cache-tree plays bigger role in terms of performance. Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]> Signed-off-by: Junio C Hamano <[email protected]>
This makes sure that cache-tree is consistent with the index. The main purpose is to catch potential problems by saving the index in unpack_trees() but the line in write_index() would also help spot missing invalidation in other code. Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]> Signed-off-by: Junio C Hamano <[email protected]>
Fix an incorrect comment in the new code added in b4da373 (unpack-trees: optimize walking same trees with cache-tree - 2018-08-18) and document about the new test variable that is enabled by default in test-lib.sh in 4592e60 (cache-tree: verify valid cache-tree in the test suite - 2018-08-18) Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]> Signed-off-by: Junio C Hamano <[email protected]>
jeffhostetler
approved these changes
Sep 17, 2018
|
In the latest "What's cooking" mail it is listed as "Will merge to 'master'." |
derrickstolee
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Oct 12, 2018
…racked files Update the package for GitForWindows to include tracing information. Includes the following Git PRs: * [36 Avoid `sane_execvp` in `git rebase` and `git stash`](microsoft/git#36) * [34 Add Trace2 regions to 'pack-objects'](microsoft/git#34) * [28 Trace2 base plus GVFS extensions](microsoft/git#28) * [33 virtualfilesystem: check if directory is included](microsoft/git#33) * [31 Revert "gvfs: add a perf test for reading the index"](microsoft/git#31) * [32 compat/poll: prepare for targeting Windows Vista](microsoft/git#32) * [22 Enable the filesystem cache (fscache) in refresh_index()](microsoft/git#22) * [27 virtualfilesystem: fix bug with symlinks being ignored](microsoft/git#27) * [23 Unpack trees with cache tree gvfs](microsoft/git#23) * [15 virtualfilesystem: don't run the virtual file system hook if the index has been redirected](microsoft/git#15) * Includes Git 2.19.0. This also includes VFS for Git updates from Kevin Willford's work: **Fix missing untracked files when created in subfolder** Add to the test to create multiple level of folders and files in those folder to make sure they show up as untracked files. Update the git for windows version that has the fix. Fixes #358
dscho
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Unpack trees with cache tree gvfs
dscho
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Unpack trees with cache tree gvfs
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Fast track optimization to unpack-trees() that utilizes the cache tree to avoid traversing parts of the tree that are the same between the two commits.