[PATCH 4 of 4] changelog: disable delta chains
Gregory Szorc
gregory.szorc at gmail.com
Sat Sep 24 15:37:19 EDT 2016
# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1474745226 25200
# Sat Sep 24 12:27:06 2016 -0700
# Node ID db0073124b876cc8b8e1201ba1b867b56c8a0448
# Parent 712281c3ab4b6e6e8ae109fa0673d4d6321d82fd
changelog: disable delta chains
This patch disables delta chains on changelogs. After this patch, new
entries on changelogs - including existing changelogs - will be stored
as the fulltext of that data (likely compressed). No delta computation
will be performed.
An overview of delta chains and data justifying this change follows.
Revlogs try to store entries as a delta against a previous entry (either
a parent revision in the case of generaldelta or the previous physical
revision when not using generaldelta). Most of the time this is the
correct thing to do: it frequently results in less CPU usage and smaller
storage.
Delta chains are most effective when the base revision being deltad
against is similar to the current data. This tends to occur naturally
for manifests and file data, since only small parts of each tend to
change with each revision. Changelogs, however, are a different story.
Changelog entries represent changesets/commits. And unless commits in a
repository are homogonous (same author, changing same files, similar
commit messages, etc), a delta from one entry to the next tends to be
relatively large compared to the size of the entry. This means that
delta chains tend to be short. How short? Here is the full vs delta
revision breakdown on some real world repos:
Repo % Full % Delta Max Length
hg 45.8 54.2 6
mozilla-central 42.4 57.6 8
mozilla-unified 42.5 57.5 17
pypy 46.1 53.9 6
python-zstandard 46.1 53.9 3
(I threw in python-zstandard as an example of a repo that is homogonous.
It contains a small Python project with changes all from the same
author.)
Contrast this with the manifest revlog for these repos, where 99+% of
revisions are deltas and delta chains run into the thousands.
So delta chains aren't as useful on changelogs. But even a short delta
chain may provide benefits. Let's measure that.
Delta chains may require less CPU to read revisions if the CPU time
spent reading smaller deltas is less than the CPU time used to
decompress larger individual entries. We can measure this via
`hg perfrevlog -c -d 1` to iterate a revlog to resolve each revision's
fulltext. Here are the results of that command on a repo using delta
chains in its changelog and on a repo without delta chains:
hg (forward)
! wall 0.407008 comb 0.410000 user 0.410000 sys 0.000000 (best of 25)
! wall 0.390061 comb 0.390000 user 0.390000 sys 0.000000 (best of 26)
hg (reverse)
! wall 0.515221 comb 0.520000 user 0.520000 sys 0.000000 (best of 19)
! wall 0.400018 comb 0.400000 user 0.390000 sys 0.010000 (best of 25)
mozilla-central (forward)
! wall 4.508296 comb 4.490000 user 4.490000 sys 0.000000 (best of 3)
! wall 4.370222 comb 4.370000 user 4.350000 sys 0.020000 (best of 3)
mozilla-central (reverse)
! wall 5.758995 comb 5.760000 user 5.720000 sys 0.040000 (best of 3)
! wall 4.346503 comb 4.340000 user 4.320000 sys 0.020000 (best of 3)
mozilla-unified (forward)
! wall 4.957088 comb 4.950000 user 4.940000 sys 0.010000 (best of 3)
! wall 4.660528 comb 4.650000 user 4.630000 sys 0.020000 (best of 3)
mozilla-unified (reverse)
! wall 6.119827 comb 6.110000 user 6.090000 sys 0.020000 (best of 3)
! wall 4.675136 comb 4.670000 user 4.670000 sys 0.000000 (best of 3)
pypy (forward)
! wall 1.231122 comb 1.240000 user 1.230000 sys 0.010000 (best of 8)
! wall 1.164896 comb 1.160000 user 1.160000 sys 0.000000 (best of 9)
pypy (reverse)
! wall 1.467049 comb 1.460000 user 1.460000 sys 0.000000 (best of 7)
! wall 1.160200 comb 1.170000 user 1.160000 sys 0.010000 (best of 9)
The data clearly shows that it takes less wall and CPU time to resolve
revisions when there are no delta chains in the changelogs, regardless
of the direction of traversal. Furthermore, not using a delta chain
means that fulltext resolution in reverse is as fast as iterating
forward. So not using delta chains on the changelog is a clear CPU win
for reading operations.
An example of a user-visible operation showing this speed-up is revset
evaluation. Here are results for
`hg perfrevset 'author(gps) or author(mpm)'`:
hg
! wall 1.655506 comb 1.660000 user 1.650000 sys 0.010000 (best of 6)
! wall 1.612723 comb 1.610000 user 1.600000 sys 0.010000 (best of 7)
mozilla-central
! wall 17.629826 comb 17.640000 user 17.600000 sys 0.040000 (best of 3)
! wall 17.311033 comb 17.300000 user 17.260000 sys 0.040000 (best of 3)
What about 00changelog.i size?
Repo Delta Chains No Delta Chains
hg 7,033,250 6,976,771
mozilla-central 82,978,748 81,574,623
mozilla-unified 88,112,349 86,702,162
pypy 20,740,699 20,659,741
The data shows that removing delta chains from the changelog makes the
changelog smaller.
Delta chains are also used during changegroup generation. This
operation essentially converts a series of revisions to one large
delta chain. And changegroup generation is smart: if the delta in
the revlog matches what the changegroup is emitting, it will reuse
the delta instead of recalculating it. We can measure the impact
removing changelog delta chains has on changegroup generation via
`hg perfchangegroupchangelog`:
hg
! wall 1.589245 comb 1.590000 user 1.590000 sys 0.000000 (best of 7)
! wall 1.788060 comb 1.790000 user 1.790000 sys 0.000000 (best of 6)
mozilla-central
! wall 17.382585 comb 17.380000 user 17.340000 sys 0.040000 (best of 3)
! wall 20.161357 comb 20.160000 user 20.120000 sys 0.040000 (best of 3)
mozilla-unified
! wall 18.722839 comb 18.720000 user 18.680000 sys 0.040000 (best of 3)
! wall 21.168075 comb 21.170000 user 21.130000 sys 0.040000 (best of 3)
pypy
! wall 4.828317 comb 4.830000 user 4.820000 sys 0.010000 (best of 3)
! wall 5.415455 comb 5.420000 user 5.410000 sys 0.010000 (best of 3)
The data shows eliminating delta chains makes the changelog part of
changegroup generation slower. This is expected since we now have to
compute deltas for revisions where we could recycle the delta before.
It is worth putting this regression into context of overall changegroup
times. Here is the rough total CPU time spent in changegroup generation
for various repos while using delta chains on the changelog:
Repo CPU Time (s) CPU Time w/ compression
hg 4.50 7.05
mozilla-central 111.1 222.0
pypy 28.68 75.5
Before compression, removing delta chains from the changegroup adds
~4.4% overhead to hg changegroup generation, 1.3% to mozilla-central,
and 2.0% to pypy. When you factor in zlib compression, these percentages
are roughly divided by 2.
While the increased CPU usage for changegroup generation is unfortunate,
I think it is acceptable because the percentage is small, server
operators (those likely impacted most by this) have other mechanisms
to mitigate CPU consumption (namely reducing zlib compression level and
pre-generated clone bundles), and because there is room to optimize this
in the future. For example, we could introduce a new changegroup format
that allowed changelog entries to be represented differently so we don't
have to compute deltas.
Removing delta chains from the changelog also opens the door to future
optimizations.
I just mentioned the possibility of a new changegroup representation for
changelog data.
Storing changelog entries as independent entities also enables more
advanced storage options. We could refactor the changelog API to
distinguish between a supported data access API and the internal
mechanism for storing that data (much like the manifest API refactor
currently under way). This would make it easier to swap in alternative
storage backends for the changelog, such as SQLite or some such. This
in turn would make it easier to support features like shallow clone,
where changesets may arrive to local storage out of DAG order.
Another possibility is enabling dictionary compression on revlogs.
Currently, each entry is compressed in isolation. Now that we have a
compressed representation of each revision and each revision has similar
structure, we would likely see benefits by using dictionary compression.
We can't do this with zlib. But it does enable easier experimentation
with compression formats that do (fun fact: experimenting with zstd
led me down a path that resulted in this patch).
There is potentially room to add a revlog flag indicating whether not
to store delta chains. This could potentially lead to more read path
optimizations to skip delta chain computation. But I'm not convinced
it is worthwhile and worth wasting a revlog flag bit on.
diff --git a/mercurial/changelog.py b/mercurial/changelog.py
--- a/mercurial/changelog.py
+++ b/mercurial/changelog.py
@@ -258,16 +258,22 @@ class changelogrevision(object):
class changelog(revlog.revlog):
def __init__(self, opener):
revlog.revlog.__init__(self, opener, "00changelog.i",
checkambig=True)
if self._initempty:
# changelogs don't benefit from generaldelta
self.version &= ~revlog.REVLOGGENERALDELTA
self._generaldelta = False
+
+ # Delta chains for changelogs tend to be very small because entries
+ # tend to be small and don't delta well with each. So disable delta
+ # chains.
+ self._storedeltachains = False
+
self._realopener = opener
self._delayed = False
self._delaybuf = None
self._divert = False
self.filteredrevs = frozenset()
def tip(self):
"""filtered version of revlog.tip"""
diff --git a/tests/test-acl.t b/tests/test-acl.t
--- a/tests/test-acl.t
+++ b/tests/test-acl.t
@@ -1504,17 +1504,17 @@ No branch acls specified
acl: path access granted: "ef1ea85a6374"
acl: branch access granted: "f9cafe1212c8" on branch "default"
acl: path access granted: "f9cafe1212c8"
acl: branch access granted: "911600dab2ae" on branch "default"
acl: path access granted: "911600dab2ae"
acl: branch access granted: "e8fc755d4d82" on branch "foobar"
acl: path access granted: "e8fc755d4d82"
updating the branch cache
- bundle2-input-part: total payload size 2101
+ bundle2-input-part: total payload size 2139
bundle2-input-part: "pushkey" (params: 4 mandatory) supported
pushing key for "phases:911600dab2ae7a9baff75958b84fe606851ce955"
bundle2-input-part: "pushkey" (params: 4 mandatory) supported
pushing key for "phases:e8fc755d4d8217ee5b0c2bb41558c40d43b92c01"
bundle2-input-bundle: 4 parts total
bundle2-output-bundle: "HG20", 3 parts total
bundle2-output-part: "reply:changegroup" (advisory) (params: 0 advisory) empty payload
bundle2-output-part: "reply:pushkey" (params: 0 advisory) empty payload
@@ -1590,17 +1590,17 @@ Branch acl deny test
acl: acl.deny not enabled
acl: branch access granted: "ef1ea85a6374" on branch "default"
acl: path access granted: "ef1ea85a6374"
acl: branch access granted: "f9cafe1212c8" on branch "default"
acl: path access granted: "f9cafe1212c8"
acl: branch access granted: "911600dab2ae" on branch "default"
acl: path access granted: "911600dab2ae"
error: pretxnchangegroup.acl hook failed: acl: user "astro" denied on branch "foobar" (changeset "e8fc755d4d82")
- bundle2-input-part: total payload size 2101
+ bundle2-input-part: total payload size 2139
bundle2-input-bundle: 4 parts total
transaction abort!
rollback completed
abort: acl: user "astro" denied on branch "foobar" (changeset "e8fc755d4d82")
no rollback information available
2:fb35475503ef
@@ -1658,17 +1658,17 @@ Branch acl empty allow test
added 4 changesets with 4 changes to 4 files (+1 heads)
calling hook pretxnchangegroup.acl: hgext.acl.hook
acl: checking access for user "astro"
acl: acl.allow.branches enabled, 0 entries for user astro
acl: acl.deny.branches not enabled
acl: acl.allow not enabled
acl: acl.deny not enabled
error: pretxnchangegroup.acl hook failed: acl: user "astro" not allowed on branch "default" (changeset "ef1ea85a6374")
- bundle2-input-part: total payload size 2101
+ bundle2-input-part: total payload size 2139
bundle2-input-bundle: 4 parts total
transaction abort!
rollback completed
abort: acl: user "astro" not allowed on branch "default" (changeset "ef1ea85a6374")
no rollback information available
2:fb35475503ef
@@ -1728,17 +1728,17 @@ Branch acl allow other
added 4 changesets with 4 changes to 4 files (+1 heads)
calling hook pretxnchangegroup.acl: hgext.acl.hook
acl: checking access for user "astro"
acl: acl.allow.branches enabled, 0 entries for user astro
acl: acl.deny.branches not enabled
acl: acl.allow not enabled
acl: acl.deny not enabled
error: pretxnchangegroup.acl hook failed: acl: user "astro" not allowed on branch "default" (changeset "ef1ea85a6374")
- bundle2-input-part: total payload size 2101
+ bundle2-input-part: total payload size 2139
bundle2-input-bundle: 4 parts total
transaction abort!
rollback completed
abort: acl: user "astro" not allowed on branch "default" (changeset "ef1ea85a6374")
no rollback information available
2:fb35475503ef
$ do_push george
@@ -1800,17 +1800,17 @@ Branch acl allow other
acl: path access granted: "ef1ea85a6374"
acl: branch access granted: "f9cafe1212c8" on branch "default"
acl: path access granted: "f9cafe1212c8"
acl: branch access granted: "911600dab2ae" on branch "default"
acl: path access granted: "911600dab2ae"
acl: branch access granted: "e8fc755d4d82" on branch "foobar"
acl: path access granted: "e8fc755d4d82"
updating the branch cache
- bundle2-input-part: total payload size 2101
+ bundle2-input-part: total payload size 2139
bundle2-input-part: "pushkey" (params: 4 mandatory) supported
pushing key for "phases:911600dab2ae7a9baff75958b84fe606851ce955"
bundle2-input-part: "pushkey" (params: 4 mandatory) supported
pushing key for "phases:e8fc755d4d8217ee5b0c2bb41558c40d43b92c01"
bundle2-input-bundle: 4 parts total
bundle2-output-bundle: "HG20", 3 parts total
bundle2-output-part: "reply:changegroup" (advisory) (params: 0 advisory) empty payload
bundle2-output-part: "reply:pushkey" (params: 0 advisory) empty payload
@@ -1893,17 +1893,17 @@ push foobar into the remote
acl: path access granted: "ef1ea85a6374"
acl: branch access granted: "f9cafe1212c8" on branch "default"
acl: path access granted: "f9cafe1212c8"
acl: branch access granted: "911600dab2ae" on branch "default"
acl: path access granted: "911600dab2ae"
acl: branch access granted: "e8fc755d4d82" on branch "foobar"
acl: path access granted: "e8fc755d4d82"
updating the branch cache
- bundle2-input-part: total payload size 2101
+ bundle2-input-part: total payload size 2139
bundle2-input-part: "pushkey" (params: 4 mandatory) supported
pushing key for "phases:911600dab2ae7a9baff75958b84fe606851ce955"
bundle2-input-part: "pushkey" (params: 4 mandatory) supported
pushing key for "phases:e8fc755d4d8217ee5b0c2bb41558c40d43b92c01"
bundle2-input-bundle: 4 parts total
bundle2-output-bundle: "HG20", 3 parts total
bundle2-output-part: "reply:changegroup" (advisory) (params: 0 advisory) empty payload
bundle2-output-part: "reply:pushkey" (params: 0 advisory) empty payload
@@ -1977,17 +1977,17 @@ Branch acl conflicting deny
added 4 changesets with 4 changes to 4 files (+1 heads)
calling hook pretxnchangegroup.acl: hgext.acl.hook
acl: checking access for user "george"
acl: acl.allow.branches not enabled
acl: acl.deny.branches enabled, 1 entries for user george
acl: acl.allow not enabled
acl: acl.deny not enabled
error: pretxnchangegroup.acl hook failed: acl: user "george" denied on branch "default" (changeset "ef1ea85a6374")
- bundle2-input-part: total payload size 2101
+ bundle2-input-part: total payload size 2139
bundle2-input-bundle: 4 parts total
transaction abort!
rollback completed
abort: acl: user "george" denied on branch "default" (changeset "ef1ea85a6374")
no rollback information available
2:fb35475503ef
User 'astro' must not be denied
@@ -2054,17 +2054,17 @@ User 'astro' must not be denied
acl: path access granted: "ef1ea85a6374"
acl: branch access granted: "f9cafe1212c8" on branch "default"
acl: path access granted: "f9cafe1212c8"
acl: branch access granted: "911600dab2ae" on branch "default"
acl: path access granted: "911600dab2ae"
acl: branch access granted: "e8fc755d4d82" on branch "foobar"
acl: path access granted: "e8fc755d4d82"
updating the branch cache
- bundle2-input-part: total payload size 2101
+ bundle2-input-part: total payload size 2139
bundle2-input-part: "pushkey" (params: 4 mandatory) supported
pushing key for "phases:911600dab2ae7a9baff75958b84fe606851ce955"
bundle2-input-part: "pushkey" (params: 4 mandatory) supported
pushing key for "phases:e8fc755d4d8217ee5b0c2bb41558c40d43b92c01"
bundle2-input-bundle: 4 parts total
bundle2-output-bundle: "HG20", 3 parts total
bundle2-output-part: "reply:changegroup" (advisory) (params: 0 advisory) empty payload
bundle2-output-part: "reply:pushkey" (params: 0 advisory) empty payload
@@ -2132,17 +2132,17 @@ Non-astro users must be denied
added 4 changesets with 4 changes to 4 files (+1 heads)
calling hook pretxnchangegroup.acl: hgext.acl.hook
acl: checking access for user "george"
acl: acl.allow.branches not enabled
acl: acl.deny.branches enabled, 1 entries for user george
acl: acl.allow not enabled
acl: acl.deny not enabled
error: pretxnchangegroup.acl hook failed: acl: user "george" denied on branch "default" (changeset "ef1ea85a6374")
- bundle2-input-part: total payload size 2101
+ bundle2-input-part: total payload size 2139
bundle2-input-bundle: 4 parts total
transaction abort!
rollback completed
abort: acl: user "george" denied on branch "default" (changeset "ef1ea85a6374")
no rollback information available
2:fb35475503ef
diff --git a/tests/test-excessive-merge.t b/tests/test-excessive-merge.t
--- a/tests/test-excessive-merge.t
+++ b/tests/test-excessive-merge.t
@@ -64,17 +64,17 @@
summary: test
$ hg debugindex --changelog
rev offset length ..... linkrev nodeid p1 p2 (re)
0 0 60 ..... 0 5e0375449e74 000000000000 000000000000 (re)
1 60 62 ..... 1 96155394af80 5e0375449e74 000000000000 (re)
2 122 62 ..... 2 92cc4c306b19 5e0375449e74 000000000000 (re)
3 184 69 ..... 3 e16a66a37edd 92cc4c306b19 96155394af80 (re)
- 4 253 29 ..... 4 2ee31f665a86 96155394af80 92cc4c306b19 (re)
+ 4 253 69 ..... 4 2ee31f665a86 96155394af80 92cc4c306b19 (re)
revision 1
$ hg manifest --debug 1
79d7492df40aa0fa093ec4209be78043c181f094 644 a
2ed2a3912a0b24502043eae84ee4b279c18b90dd 644 b
revision 2
$ hg manifest --debug 2
2ed2a3912a0b24502043eae84ee4b279c18b90dd 644 a
diff --git a/tests/test-ssh-bundle1.t b/tests/test-ssh-bundle1.t
--- a/tests/test-ssh-bundle1.t
+++ b/tests/test-ssh-bundle1.t
@@ -55,18 +55,18 @@ non-existent absolute path
remote: abort: repository /$TESTTMP/nonexistent not found!
abort: no suitable response from remote hg!
[255]
clone remote via stream
$ hg clone -e "python \"$TESTDIR/dummyssh\"" --uncompressed ssh://user@dummy/remote local-stream
streaming all changes
- 4 files to transfer, 615 bytes of data
- transferred 615 bytes in * seconds (*) (glob)
+ 4 files to transfer, 602 bytes of data
+ transferred 602 bytes in * seconds (*) (glob)
searching for changes
no changes found
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd local-stream
$ hg verify
checking changesets
checking manifests
@@ -77,18 +77,18 @@ clone remote via stream
default 0:1160648e36ce
$ cd ..
clone bookmarks via stream
$ hg -R local-stream book mybook
$ hg clone -e "python \"$TESTDIR/dummyssh\"" --uncompressed ssh://user@dummy/local-stream stream2
streaming all changes
- 4 files to transfer, 615 bytes of data
- transferred 615 bytes in * seconds (*) (glob)
+ 4 files to transfer, 602 bytes of data
+ transferred 602 bytes in * seconds (*) (glob)
searching for changes
no changes found
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd stream2
$ hg book
mybook 0:1160648e36ce
$ cd ..
diff --git a/tests/test-ssh.t b/tests/test-ssh.t
--- a/tests/test-ssh.t
+++ b/tests/test-ssh.t
@@ -49,18 +49,18 @@ non-existent absolute path
remote: abort: repository $TESTTMP/nonexistent not found!
abort: no suitable response from remote hg!
[255]
clone remote via stream
$ hg clone -e "python \"$TESTDIR/dummyssh\"" --uncompressed ssh://user@dummy/remote local-stream
streaming all changes
- 4 files to transfer, 615 bytes of data
- transferred 615 bytes in * seconds (*) (glob)
+ 4 files to transfer, 602 bytes of data
+ transferred 602 bytes in * seconds (*) (glob)
searching for changes
no changes found
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd local-stream
$ hg verify
checking changesets
checking manifests
@@ -71,18 +71,18 @@ clone remote via stream
default 0:1160648e36ce
$ cd ..
clone bookmarks via stream
$ hg -R local-stream book mybook
$ hg clone -e "python \"$TESTDIR/dummyssh\"" --uncompressed ssh://user@dummy/local-stream stream2
streaming all changes
- 4 files to transfer, 615 bytes of data
- transferred 615 bytes in * seconds (*) (glob)
+ 4 files to transfer, 602 bytes of data
+ transferred 602 bytes in * seconds (*) (glob)
searching for changes
no changes found
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd stream2
$ hg book
mybook 0:1160648e36ce
$ cd ..
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