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The final part of integrating the compression manager APIs into revlog storage is the plumbing for repositories to advertise they are using non-zlib storage and for revlogs to instantiate a non-zlib compression engine. The main intent of the compression manager work was to zstd all of the things. Adding zstd to revlogs has proved to be more involved than other places because revlogs are... special. Very small inputs and the use of delta chains (which are themselves a form of compression) are a completely different use case from streaming compression, which bundles and the wire protocol employ. I've conducted numerous experiments with zstd in revlogs and have yet to formalize compression settings and a storage architecture that I'm confident I won't regret later. In other words, I'm not yet ready to commit to a new mechanism for using zstd - or any other compression format - in revlogs. That being said, having some support for zstd (and other compression formats) in revlogs in core is beneficial. It can allow others to conduct experiments. This patch introduces *highly experimental* support for non-zlib compression formats in revlogs. Introduced is a config option to control which compression engine to use. Also introduced is a namespace of "exp-compression-*" requirements to denote support for non-zlib compression in revlogs. I've prefixed the namespace with "exp-" (short for "experimental") because I'm not confident of the requirements "schema" and in no way want to give the illusion of supporting these requirements in the future. I fully intend to drop support for these requirements once we figure out what we're doing with zstd in revlogs. A good portion of the patch is teaching the requirements system about registered compression engines and passing the requested compression engine as an opener option so revlogs can instantiate the proper compression engine for new operations. That's a verbose way of saying "we can now use zstd in revlogs!" On an `hg pull` conversion of the mozilla-unified repo with no extra redelta settings (like aggressivemergedeltas), we can see the impact of zstd vs zlib in revlogs: $ hg perfrevlogchunks -c ! chunk ! wall 2.032052 comb 2.040000 user 1.990000 sys 0.050000 (best of 5) ! wall 1.866360 comb 1.860000 user 1.820000 sys 0.040000 (best of 6) ! chunk batch ! wall 1.877261 comb 1.870000 user 1.860000 sys 0.010000 (best of 6) ! wall 1.705410 comb 1.710000 user 1.690000 sys 0.020000 (best of 6) $ hg perfrevlogchunks -m ! chunk ! wall 2.721427 comb 2.720000 user 2.640000 sys 0.080000 (best of 4) ! wall 2.035076 comb 2.030000 user 1.950000 sys 0.080000 (best of 5) ! chunk batch ! wall 2.614561 comb 2.620000 user 2.580000 sys 0.040000 (best of 4) ! wall 1.910252 comb 1.910000 user 1.880000 sys 0.030000 (best of 6) $ hg perfrevlog -c -d 1 ! wall 4.812885 comb 4.820000 user 4.800000 sys 0.020000 (best of 3) ! wall 4.699621 comb 4.710000 user 4.700000 sys 0.010000 (best of 3) $ hg perfrevlog -m -d 1000 ! wall 34.252800 comb 34.250000 user 33.730000 sys 0.520000 (best of 3) ! wall 24.094999 comb 24.090000 user 23.320000 sys 0.770000 (best of 3) Only modest wins for the changelog. But manifest reading is significantly faster. What's going on? One reason might be data volume. zstd decompresses faster. So given more bytes, it will put more distance between it and zlib. Another reason is size. In the current design, zstd revlogs are *larger*: debugcreatestreamclonebundle (size in bytes) zlib: 1,638,852,492 zstd: 1,680,601,332 I haven't investigated this fully, but I reckon a significant cause of larger revlogs is that the zstd frame/header has more bytes than zlib's. For very small inputs or data that doesn't compress well, we'll tend to store more uncompressed chunks than with zlib (because the compressed size isn't smaller than original). This will make revlog reading faster because it is doing less decompression. Moving on to bundle performance: $ hg bundle -a -t none-v2 (total CPU time) zlib: 102.79s zstd: 97.75s So, marginal CPU decrease for reading all chunks in all revlogs (this is somewhat disappointing). $ hg bundle -a -t <engine>-v2 (total CPU time) zlib: 191.59s zstd: 115.36s This last test effectively measures the difference between zlib->zlib and zstd->zstd for revlogs to bundle. This is a rough approximation of what a server does during `hg clone`. There are some promising results for zstd. But not enough for me to feel comfortable advertising it to users. We'll get there...
79 lines
1.9 KiB
Perl
79 lines
1.9 KiB
Perl
A new repository uses zlib storage, which doesn't need a requirement
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$ hg init default
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$ cd default
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$ cat .hg/requires
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dotencode
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fncache
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generaldelta
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revlogv1
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store
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$ touch foo
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$ hg -q commit -A -m 'initial commit with a lot of repeated repeated repeated text to trigger compression'
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$ hg debugrevlog -c | grep 0x78
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0x78 (x) : 1 (100.00%)
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0x78 (x) : 110 (100.00%)
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$ cd ..
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Unknown compression engine to format.compression aborts
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$ hg --config experimental.format.compression=unknown init unknown
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abort: compression engine unknown defined by experimental.format.compression not available
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(run "hg debuginstall" to list available compression engines)
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[255]
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A requirement specifying an unknown compression engine results in bail
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$ hg init unknownrequirement
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$ cd unknownrequirement
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$ echo exp-compression-unknown >> .hg/requires
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$ hg log
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abort: repository requires features unknown to this Mercurial: exp-compression-unknown!
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(see https://mercurial-scm.org/wiki/MissingRequirement for more information)
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[255]
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$ cd ..
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#if zstd
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$ hg --config experimental.format.compression=zstd init zstd
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$ cd zstd
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$ cat .hg/requires
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dotencode
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exp-compression-zstd
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fncache
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generaldelta
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revlogv1
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store
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$ touch foo
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$ hg -q commit -A -m 'initial commit with a lot of repeated repeated repeated text'
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$ hg debugrevlog -c | grep 0x28
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0x28 : 1 (100.00%)
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0x28 : 98 (100.00%)
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$ cd ..
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Specifying a new format.compression on an existing repo won't introduce data
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with that engine or a requirement
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$ cd default
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$ touch bar
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$ hg --config experimental.format.compression=zstd -q commit -A -m 'add bar with a lot of repeated repeated repeated text'
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$ cat .hg/requires
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dotencode
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fncache
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generaldelta
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revlogv1
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store
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$ hg debugrevlog -c | grep 0x78
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0x78 (x) : 2 (100.00%)
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0x78 (x) : 199 (100.00%)
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#endif
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