git/pack-objects.h, branch v2.19.0

Merge branch 'nd/pack-deltify-regression-fix'

2018-08-22T18:17:05Z

In a recent update in 2.18 era, "git pack-objects" started producing a larger than necessary packfiles by missing opportunities to use large deltas. * nd/pack-deltify-regression-fix: pack-objects: fix performance issues on packing large deltas

Add missing includes and forward declarations

2018-08-15T18:52:09Z

I looped over the toplevel header files, creating a temporary two-line C program for each consisting of #include "git-compat-util.h" #include $HEADER This patch is the result of manually fixing errors in compiling those tiny programs. Signed-off-by: Elijah Newren Signed-off-by: Junio C Hamano

pack-objects: fix performance issues on packing large deltas

2018-07-23T17:21:29Z

Let's start with some background about oe_delta_size() and oe_set_delta_size(). If you already know, skip the next paragraph. These two are added in 0aca34e826 (pack-objects: shrink delta_size field in struct object_entry - 2018-04-14) to help reduce 'struct object_entry' size. The delta size field in this struct is reduced to only contain max 1MB. So if any new delta is produced and larger than 1MB, it's dropped because we can't really save such a large size anywhere. Fallback is provided in case existing packfiles already have large deltas, then we can retrieve it from the pack. While this should help small machines repacking large repos without large deltas (i.e. less memory pressure), dropping large deltas during the delta selection process could end up with worse pack files. And if existing packfiles already have >1MB delta and pack-objects is instructed to not reuse deltas, all of them will be dropped on the floor, and the resulting pack would be definitely bigger. There is also a regression in terms of CPU/IO if we have large on-disk deltas because fallback code needs to parse the pack every time the delta size is needed and just access to the mmap'd pack data is enough for extra page faults when memory is under pressure. Both of these issues were reported on the mailing list. Here's some numbers for comparison. Version Pack (MB) MaxRSS(kB) Time (s) ------- --------- ---------- -------- 2.17.0 5498 43513628 2494.85 2.18.0 10531 40449596 4168.94 This patch provides a better fallback that is - cheaper in terms of cpu and io because we won't have to read existing pack files as much - better in terms of pack size because the pack heuristics is back to 2.17.0 time, we do not drop large deltas at all If we encounter any delta (on-disk or created during try_delta phase) that is larger than the 1MB limit, we stop using delta_size_ field for this because it can't contain such size anyway. A new array of delta size is dynamically allocated and can hold all the deltas that 2.17.0 can. This array only contains delta sizes that delta_size_ can't contain. With this, we do not have to drop deltas in try_delta() anymore. Of course the downside is we use slightly more memory, even compared to 2.17.0. But since this is considered an uncommon case, a bit more memory consumption should not be a problem. Delta size limit is also raised from 1MB to 16MB to better cover common case and avoid that extra memory consumption (99.999% deltas in this reported repo are under 12MB; Jeff noted binary artifacts topped out at about 3MB in some other private repos). Other fields are shuffled around to keep this struct packed tight. We don't use more memory in common case even with this limit update. A note about thread synchronization. Since this code can be run in parallel during delta searching phase, we need a mutex. The realloc part in packlist_alloc() is not protected because it only happens during the object counting phase, which is always single-threaded. Access to e->delta_size_ (and by extension pack->delta_size[e - pack->objects]) is unprotected as before, the thread scheduler in pack-objects must make sure "e" is never updated by two different threads. The area under the new lock is as small as possible, avoiding locking at all in common case, since lock contention with high thread count could be expensive (most blobs are small enough that delta compute time is short and we end up taking the lock very often). The previous attempt to always hold a lock in oe_delta_size() and oe_set_delta_size() increases execution time by 33% when repacking linux.git with with 40 threads. Reported-by: Elijah Newren Helped-by: Elijah Newren Helped-by: Jeff King Signed-off-by: Nguyễn Thái Ngọc Duy Signed-off-by: Junio C Hamano

Merge branch 'nd/pack-objects-pack-struct'

2018-05-23T05:38:19Z

"git pack-objects" needs to allocate tons of "struct object_entry" while doing its work, and shrinking its size helps the performance quite a bit. * nd/pack-objects-pack-struct: ci: exercise the whole test suite with uncommon code in pack-objects pack-objects: reorder members to shrink struct object_entry pack-objects: shrink delta_size field in struct object_entry pack-objects: shrink size field in struct object_entry pack-objects: clarify the use of object_entry::size pack-objects: don't check size when the object is bad pack-objects: shrink z_delta_size field in struct object_entry pack-objects: refer to delta objects by index instead of pointer pack-objects: move in_pack out of struct object_entry pack-objects: move in_pack_pos out of struct object_entry pack-objects: use bitfield for object_entry::depth pack-objects: use bitfield for object_entry::dfs_state pack-objects: turn type and in_pack_type to bitfields pack-objects: a bit of document about struct object_entry read-cache.c: make $GIT_TEST_SPLIT_INDEX boolean

gc --auto: exclude base pack if not enough mem to "repack -ad"

2018-04-16T04:52:29Z

pack-objects could be a big memory hog especially on large repos, everybody knows that. The suggestion to stick a .keep file on the giant base pack to avoid this problem is also known for a long time. Recent patches add an option to do just this, but it has to be either configured or activated manually. This patch lets `git gc --auto` activate this mode automatically when it thinks `repack -ad` will use a lot of memory and start affecting the system due to swapping or flushing OS cache. gc --auto decides to do this based on an estimation of pack-objects memory usage, which is quite accurate at least for the heap part, and whether that fits in half of system memory (the assumption here is for desktop environment where there are many other applications running). This mechanism only kicks in if gc.bigBasePackThreshold is not configured. If it is, it is assumed that the user already knows what they want. Signed-off-by: Nguyễn Thái Ngọc Duy Signed-off-by: Junio C Hamano

pack-objects: reorder members to shrink struct object_entry

2018-04-16T03:38:59Z

Previous patches leave lots of holes and padding in this struct. This patch reorders the members and shrinks the struct down to 80 bytes (from 136 bytes on 64-bit systems, before any field shrinking is done) with 16 bits to spare (and a couple more in in_pack_header_size when we really run out of bits). This is the last in a series of memory reduction patches (see "pack-objects: a bit of document about struct object_entry" for the first one). Overall they've reduced repack memory size on linux-2.6.git from 3.747G to 3.424G, or by around 320M, a decrease of 8.5%. The runtime of repack has stayed the same throughout this series. Ævar's testing on a big monorepo he has access to (bigger than linux-2.6.git) has shown a 7.9% reduction, so the overall expected improvement should be somewhere around 8%. See 87po42cwql.fsf@evledraar.gmail.com on-list (https://public-inbox.org/git/87po42cwql.fsf@evledraar.gmail.com/) for more detailed numbers and a test script used to produce the numbers cited above. Signed-off-by: Nguyễn Thái Ngọc Duy Signed-off-by: Junio C Hamano

pack-objects: shrink delta_size field in struct object_entry

2018-04-16T03:38:59Z

Allowing a delta size of 64 bits is crazy. Shrink this field down to 20 bits with one overflow bit. If we find an existing delta larger than 1MB, we do not cache delta_size at all and will get the value from oe_size(), potentially from disk if it's larger than 4GB. Note, since DELTA_SIZE() is used in try_delta() code, it must be thread-safe. Luckily oe_size() does guarantee this so we it is thread-safe. Signed-off-by: Nguyễn Thái Ngọc Duy Signed-off-by: Junio C Hamano

pack-objects: shrink size field in struct object_entry

2018-04-16T03:38:59Z

It's very very rare that an uncompressed object is larger than 4GB (partly because Git does not handle those large files very well to begin with). Let's optimize it for the common case where object size is smaller than this limit. Shrink size field down to 31 bits and one overflow bit. If the size is too large, we read it back from disk. As noted in the previous patch, we need to return the delta size instead of canonical size when the to-be-reused object entry type is a delta instead of a canonical one. Add two compare helpers that can take advantage of the overflow bit (e.g. if the file is 4GB+, chances are it's already larger than core.bigFileThreshold and there's no point in comparing the actual value). Another note about oe_get_size_slow(). This function MUST be thread safe because SIZE() macro is used inside try_delta() which may run in parallel. Outside parallel code, no-contention locking should be dirt cheap (or insignificant compared to i/o access anyway). To exercise this code, it's best to run the test suite with something like make test GIT_TEST_OE_SIZE=4 which forces this code on all objects larger than 3 bytes. Signed-off-by: Nguyễn Thái Ngọc Duy Signed-off-by: Junio C Hamano

pack-objects: clarify the use of object_entry::size

2018-04-16T03:38:59Z

While this field most of the time contains the canonical object size, there is one case it does not: when we have found that the base object of the delta in question is also to be packed, we will very happily reuse the delta by copying it over instead of regenerating the new delta. "size" in this case will record the delta size, not canonical object size. Later on in write_reuse_object(), we reconstruct the delta header and "size" is used for this purpose. When this happens, the "type" field contains a delta type instead of a canonical type. Highlight this in the code since it could be tricky to see. Signed-off-by: Nguyễn Thái Ngọc Duy Signed-off-by: Junio C Hamano

pack-objects: shrink z_delta_size field in struct object_entry

2018-04-16T03:38:58Z

We only cache deltas when it's smaller than a certain limit. This limit defaults to 1000 but save its compressed length in a 64-bit field. Shrink that field down to 20 bits, so you can only cache 1MB deltas. Larger deltas must be recomputed at when the pack is written down. Signed-off-by: Nguyễn Thái Ngọc Duy Signed-off-by: Junio C Hamano