[PATCH v2 5/5] reftable/blocksource: use mmap to read tables
- From
Patrick Steinhardt <ps@pks.im>
- Date
- Jan 11, 2024, 10:06 UTC
- Message-ID
- <e53a20c8e1a5f2169da4fddf2d430d584a7731d1.1704966670.git.ps@pks.im>
- In-Reply-To
- <cover.1704966670.git.ps@pks.im>
The blocksource interface provides an interface to read blocks from a reftable table. This interface is implemented using read(3P) calls on the underlying file descriptor. While this works alright, this pattern is very inefficient when repeatedly querying the reftable stack for one or more refs. This inefficiency can mostly be attributed to the fact that we often need to re-read the same blocks over and over again, and every single time we need to call read(3P) again.
A natural fit in this context is to use mmap(3P) instead of read(3P), which has a bunch of benefits:
- We do not need to come up with a caching strategy for some of the
blocks as this will be handled by the kernel already. - We can avoid the overhead of having to call into the read(3P)
syscall repeatedly. - We do not need to allocate returned blocks repeatedly, but can
instead hand out pointers into the mmapped region directly.Using mmap comes with a significant drawback on Windows though, because mmapped files cannot be deleted and neither is it possible to rename files onto an mmapped file. But for one, the reftable library gracefully handles the case where auto-compaction cannot delete a still-open stack already and ignores any such errors. Also, `reftable_stack_clean()` will prune stale tables which are not referenced by "tables.list" anymore so that those files can eventually be pruned. And second, we never rewrite already-written stacks, so it does not matter that we cannot rename a file over an mmaped file, either.
Another unfortunate property of mmap is that it is not supported by all systems. But given that the size of reftables should typically be rather limited (megabytes at most in the vast majority of repositories), we can use the fallback implementation provided by `git_mmap()` which reads the whole file into memory instead. This is the same strategy that the "packed" backend uses.
While this change doesn't significantly improve performance in the case where we're seeking through stacks once (like e.g. git-for-each-ref(1) would). But it does speed up usecases where there is lots of random access to refs, e.g. when writing. The following benchmark demonstrates these savings with git-update-ref(1) creating N refs in an otherwise empty repository:
Benchmark 1: update-ref: create many refs (refcount = 1, revision = HEAD~)
Time (mean ± σ): 5.1 ms ± 0.2 ms [User: 2.5 ms, System: 2.5 ms]
Range (min … max): 4.8 ms … 7.1 ms 111 runs Benchmark 2: update-ref: create many refs (refcount = 100, revision = HEAD~)
Time (mean ± σ): 14.8 ms ± 0.5 ms [User: 7.1 ms, System: 7.5 ms]
Range (min … max): 14.1 ms … 18.7 ms 84 runs Benchmark 3: update-ref: create many refs (refcount = 10000, revision = HEAD~)
Time (mean ± σ): 926.4 ms ± 5.6 ms [User: 448.5 ms, System: 477.7 ms]
Range (min … max): 920.0 ms … 936.1 ms 10 runs Benchmark 4: update-ref: create many refs (refcount = 1, revision = HEAD)
Time (mean ± σ): 5.0 ms ± 0.2 ms [User: 2.4 ms, System: 2.5 ms]
Range (min … max): 4.7 ms … 5.4 ms 111 runs Benchmark 5: update-ref: create many refs (refcount = 100, revision = HEAD)
Time (mean ± σ): 10.5 ms ± 0.2 ms [User: 5.0 ms, System: 5.3 ms]
Range (min … max): 10.0 ms … 10.9 ms 93 runs Benchmark 6: update-ref: create many refs (refcount = 10000, revision = HEAD)
Time (mean ± σ): 529.6 ms ± 9.1 ms [User: 268.0 ms, System: 261.4 ms]
Range (min … max): 522.4 ms … 547.1 ms 10 runs Summary
update-ref: create many refs (refcount = 1, revision = HEAD) ran
1.01 ± 0.06 times faster than update-ref: create many refs (refcount = 1, revision = HEAD~)
2.08 ± 0.07 times faster than update-ref: create many refs (refcount = 100, revision = HEAD)
2.95 ± 0.14 times faster than update-ref: create many refs (refcount = 100, revision = HEAD~)
105.33 ± 3.76 times faster than update-ref: create many refs (refcount = 10000, revision = HEAD)
184.24 ± 5.89 times faster than update-ref: create many refs (refcount = 10000, revision = HEAD~)Theoretically, we could also replicate the strategy of the "packed" backend where small tables are read into memory instead of using mmap. Benchmarks did not confirm that this has a performance benefit though.
Signed-off-by: Patrick Steinhardt <ps@pks.im> --- reftable/blocksource.c | 22 +++++++--------------- 1 file changed, 7 insertions(+), 15 deletions(-)
diff --git a/reftable/blocksource.c b/reftable/blocksource.c index 1e2fb5e9fd..8c41e3c70f 100644 --- a/reftable/blocksource.c +++ b/reftable/blocksource.c @@ -76,8 +76,8 @@ struct reftable_block_source malloc_block_source(void) } struct file_block_source { - int fd; uint64_t size; + unsigned char *data; }; static uint64_t file_size(void *b) @@ -87,19 +87,12 @@ static uint64_t file_size(void *b) static void file_return_block(void *b, struct reftable_block *dest) { - if (dest->len) - memset(dest->data, 0xff, dest->len); - reftable_free(dest->data); } -static void file_close(void *b) +static void file_close(void *v) { - int fd = ((struct file_block_source *)b)->fd; - if (fd > 0) { - close(fd); - ((struct file_block_source *)b)->fd = 0; - } - + struct file_block_source *b = v; + munmap(b->data, b->size); reftable_free(b); } @@ -108,9 +101,7 @@ static int file_read_block(void *v, struct reftable_block *dest, uint64_t off, { struct file_block_source *b = v; assert(off + size <= b->size); - dest->data = reftable_malloc(size); - if (pread_in_full(b->fd, dest->data, size, off) != size) - return -1; + dest->data = b->data + off; dest->len = size; return size; } @@ -143,7 +134,8 @@ int reftable_block_source_from_file(struct reftable_block_source *bs, p = reftable_calloc(sizeof(*p)); p->size = st.st_size; - p->fd = fd; + p->data = xmmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0); + close(fd); assert(!bs->ops); bs->ops = &file_vtable;
-- 2.43.GIT