localrepo: experimental support for non-zlib revlog compression
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...
#require execbit
$ cat <<EOF >> $HGRCPATH
> [extensions]
> autodiff = $TESTDIR/autodiff.py
> [diff]
> nodates = 1
> EOF
$ hg init repo
$ cd repo
make a combination of new, changed and deleted file
$ echo regular > regular
$ echo rmregular > rmregular
$ $PYTHON -c "file('bintoregular', 'wb').write('\0')"
$ touch rmempty
$ echo exec > exec
$ chmod +x exec
$ echo rmexec > rmexec
$ chmod +x rmexec
$ echo setexec > setexec
$ echo unsetexec > unsetexec
$ chmod +x unsetexec
$ echo binary > binary
$ $PYTHON -c "file('rmbinary', 'wb').write('\0')"
$ hg ci -Am addfiles
adding binary
adding bintoregular
adding exec
adding regular
adding rmbinary
adding rmempty
adding rmexec
adding rmregular
adding setexec
adding unsetexec
$ echo regular >> regular
$ echo newregular >> newregular
$ rm rmempty
$ touch newempty
$ rm rmregular
$ echo exec >> exec
$ echo newexec > newexec
$ echo bintoregular > bintoregular
$ chmod +x newexec
$ rm rmexec
$ chmod +x setexec
$ chmod -x unsetexec
$ $PYTHON -c "file('binary', 'wb').write('\0\0')"
$ $PYTHON -c "file('newbinary', 'wb').write('\0')"
$ rm rmbinary
$ hg addremove -s 0
adding newbinary
adding newempty
adding newexec
adding newregular
removing rmbinary
removing rmempty
removing rmexec
removing rmregular
git=no: regular diff for all files
$ hg autodiff --git=no
diff -r a66d19b9302d binary
Binary file binary has changed
diff -r a66d19b9302d bintoregular
Binary file bintoregular has changed
diff -r a66d19b9302d exec
--- a/exec
+++ b/exec
@@ -1,1 +1,2 @@
exec
+exec
diff -r a66d19b9302d newbinary
Binary file newbinary has changed
diff -r a66d19b9302d newexec
--- /dev/null
+++ b/newexec
@@ -0,0 +1,1 @@
+newexec
diff -r a66d19b9302d newregular
--- /dev/null
+++ b/newregular
@@ -0,0 +1,1 @@
+newregular
diff -r a66d19b9302d regular
--- a/regular
+++ b/regular
@@ -1,1 +1,2 @@
regular
+regular
diff -r a66d19b9302d rmbinary
Binary file rmbinary has changed
diff -r a66d19b9302d rmexec
--- a/rmexec
+++ /dev/null
@@ -1,1 +0,0 @@
-rmexec
diff -r a66d19b9302d rmregular
--- a/rmregular
+++ /dev/null
@@ -1,1 +0,0 @@
-rmregular
git=yes: git diff for single regular file
$ hg autodiff --git=yes regular
diff --git a/regular b/regular
--- a/regular
+++ b/regular
@@ -1,1 +1,2 @@
regular
+regular
git=auto: regular diff for regular files and non-binary removals
$ hg autodiff --git=auto regular newregular rmregular rmexec
diff -r a66d19b9302d newregular
--- /dev/null
+++ b/newregular
@@ -0,0 +1,1 @@
+newregular
diff -r a66d19b9302d regular
--- a/regular
+++ b/regular
@@ -1,1 +1,2 @@
regular
+regular
diff -r a66d19b9302d rmexec
--- a/rmexec
+++ /dev/null
@@ -1,1 +0,0 @@
-rmexec
diff -r a66d19b9302d rmregular
--- a/rmregular
+++ /dev/null
@@ -1,1 +0,0 @@
-rmregular
$ for f in exec newexec setexec unsetexec binary newbinary newempty rmempty rmbinary bintoregular; do
> echo
> echo '% git=auto: git diff for' $f
> hg autodiff --git=auto $f
> done
% git=auto: git diff for exec
diff -r a66d19b9302d exec
--- a/exec
+++ b/exec
@@ -1,1 +1,2 @@
exec
+exec
% git=auto: git diff for newexec
diff --git a/newexec b/newexec
new file mode 100755
--- /dev/null
+++ b/newexec
@@ -0,0 +1,1 @@
+newexec
% git=auto: git diff for setexec
diff --git a/setexec b/setexec
old mode 100644
new mode 100755
% git=auto: git diff for unsetexec
diff --git a/unsetexec b/unsetexec
old mode 100755
new mode 100644
% git=auto: git diff for binary
diff --git a/binary b/binary
index a9128c283485202893f5af379dd9beccb6e79486..09f370e38f498a462e1ca0faa724559b6630c04f
GIT binary patch
literal 2
Jc${Nk0000200961
% git=auto: git diff for newbinary
diff --git a/newbinary b/newbinary
new file mode 100644
index e69de29bb2d1d6434b8b29ae775ad8c2e48c5391..f76dd238ade08917e6712764a16a22005a50573d
GIT binary patch
literal 1
Ic${MZ000310RR91
% git=auto: git diff for newempty
diff --git a/newempty b/newempty
new file mode 100644
% git=auto: git diff for rmempty
diff --git a/rmempty b/rmempty
deleted file mode 100644
% git=auto: git diff for rmbinary
diff --git a/rmbinary b/rmbinary
deleted file mode 100644
index f76dd238ade08917e6712764a16a22005a50573d..e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
GIT binary patch
literal 0
Hc$@<O00001
% git=auto: git diff for bintoregular
diff --git a/bintoregular b/bintoregular
index f76dd238ade08917e6712764a16a22005a50573d..9c42f2b6427d8bf034b7bc23986152dc01bfd3ab
GIT binary patch
literal 13
Uc$`bh%qz(+N=+}#Ni5<5043uE82|tP
git=warn: regular diff with data loss warnings
$ hg autodiff --git=warn
diff -r a66d19b9302d binary
Binary file binary has changed
diff -r a66d19b9302d bintoregular
Binary file bintoregular has changed
diff -r a66d19b9302d exec
--- a/exec
+++ b/exec
@@ -1,1 +1,2 @@
exec
+exec
diff -r a66d19b9302d newbinary
Binary file newbinary has changed
diff -r a66d19b9302d newexec
--- /dev/null
+++ b/newexec
@@ -0,0 +1,1 @@
+newexec
diff -r a66d19b9302d newregular
--- /dev/null
+++ b/newregular
@@ -0,0 +1,1 @@
+newregular
diff -r a66d19b9302d regular
--- a/regular
+++ b/regular
@@ -1,1 +1,2 @@
regular
+regular
diff -r a66d19b9302d rmbinary
Binary file rmbinary has changed
diff -r a66d19b9302d rmexec
--- a/rmexec
+++ /dev/null
@@ -1,1 +0,0 @@
-rmexec
diff -r a66d19b9302d rmregular
--- a/rmregular
+++ /dev/null
@@ -1,1 +0,0 @@
-rmregular
data lost for: binary
data lost for: bintoregular
data lost for: newbinary
data lost for: newempty
data lost for: newexec
data lost for: rmbinary
data lost for: rmempty
data lost for: setexec
data lost for: unsetexec
git=abort: fail on execute bit change
$ hg autodiff --git=abort regular setexec
abort: losing data for setexec
[255]
git=abort: succeed on regular file
$ hg autodiff --git=abort regular
diff -r a66d19b9302d regular
--- a/regular
+++ b/regular
@@ -1,1 +1,2 @@
regular
+regular
$ cd ..