view tests/test-ui-verbosity.py.out @ 51681:522b4d729e89

mmap: populate the mapping by default

Without pre-population, accessing all data through a mmap can result in many
pagefault, reducing performance significantly. If the mmap is prepopulated, the
performance can no longer get slower than a full read.
(See benchmark number below)

In some cases were very few data is read, prepopulating can be overkill and
slower than populating on access (through page fault). So that behavior can be
controlled when the caller can pre-determine the best behavior.
(See benchmark number below)

In addition, testing with populating in a secondary thread yield great result
combining the best of each approach. This might be implemented in later
changesets.

In all cases, using mmap has a great effect on memory usage when many processes
run in parallel on the same machine.

### Benchmarks

# What did I run

A couple of month back I ran a large benchmark campaign to assess the impact of
various approach for using mmap with the revlog (and other files), it
highlighted a few benchmarks that capture the impact of the changes well. So to
validate this change I checked the following:

- log command displaying various revisions
  (read the changelog index)
- log command displaying the patch of listed revisions
  (read the changelog index, the manifest index and a few files indexes)
- unbundling a few revisions
  (read and write changelog, manifest and few files indexes, and walk the graph
  to update some cache)
- pushing a few revisions
  (read and write changelog, manifest and few files indexes, walk the graph to
  update some cache, performs various accesses locally and remotely during
  discovery)

Benchmarks were run using the default module policy (c+py) and the rust one. No
significant difference were found between the two implementation, so we will
present result using the default policy (unless otherwise specified).

I ran them on a few repositories :

- mercurial: a "public changeset only" copy of mercurial from 2018-08-01 using
  zstd compression and sparse-revlog

- pypy: a copy of pypy from 2018-08-01 using zstd compression and sparse-revlog

- netbeans: a copy of netbeans from 2018-08-01 using zstd compression and
  sparse-revlog

- mozilla-try: a copy of mozilla-try from 2019-02-18 using zstd compression and
  sparse-revlog

- mozilla-try persistent-nodemap: Same as the above but with a persistent
  nodemap. Used for the log --patch benchmark only

# Results

For the smaller repositories (mercurial, pypy), the impact of mmap is almost
imperceptible, other cost dominating the operation. The impact of prepopulating
is undiscernible in the benchmark we ran.

For larger repositories the benchmark support explanation given above:

On netbeans, the log can be about 1% faster without repopulation (for a
difference < 100ms) but unbundle becomes a bit slower, even when small.

### data-env-vars.name                             = netbeans-2018-08-01-zstd-sparse-revlog
  # benchmark.name                                 = hg.command.unbundle
  # benchmark.variants.issue6528                   = disabled
  # benchmark.variants.reuse-external-delta-parent = yes
  # benchmark.variants.revs                        = any-1-extra-rev
  # benchmark.variants.source                      = unbundle
  # benchmark.variants.verbosity                   = quiet
with-populate: 0.240157
no-populate:   0.265087  (+10.38%, +0.02)
  # benchmark.variants.revs                        = any-100-extra-rev
with-populate: 1.459518
no-populate:   1.481290   (+1.49%, +0.02)
 ## benchmark.name                                 = hg.command.push
  # benchmark.variants.explicit-rev                = none
  # benchmark.variants.issue6528                   = disabled
  # benchmark.variants.protocol                    = ssh
  # benchmark.variants.reuse-external-delta-parent = yes
  # benchmark.variants.revs                        = any-1-extra-rev
with-populate: 0.771919
no-populate:   0.792025   (+2.60%, +0.02)
  # benchmark.variants.revs                        = any-100-extra-rev
with-populate: 1.459518
no-populate:   1.481290   (+1.49%, +0.02)


For mozilla-try, the "slow down" from pre-populate for small `hg log` is more
visible, but still small in absolute time. (using rust value for the persistent
nodemap value to be relevant).

### data-env-vars.name            = mozilla-try-2019-02-18-ds2-pnm
  # benchmark.name                = hg.command.log
  # bin-env-vars.hg.flavor        = rust
  # benchmark.variants.patch      = yes
  # benchmark.variants.limit-rev  = 1
with-populate: 0.237813
no-populate:   0.229452   (-3.52%, -0.01)
  # benchmark.variants.limit-rev  = 10
  # benchmark.variants.patch      = yes
with-populate: 1.213578
no-populate:   1.205189
### data-env-vars.name            = mozilla-try-2019-02-18-zstd-sparse-revlog
  # benchmark.variants.limit-rev  = 1000
  # benchmark.variants.patch      = no
  # benchmark.variants.rev        = tip
with-populate: 0.198607
no-populate:   0.195038   (-1.80%, -0.00)

However pre-populating provide a significant boost on more complex operations
like unbundle or push:

### data-env-vars.name                             = mozilla-try-2019-02-18-zstd-sparse-revlog
  # benchmark.name                                 = hg.command.push
  # benchmark.variants.explicit-rev                = none
  # benchmark.variants.issue6528                   = disabled
  # benchmark.variants.protocol                    = ssh
  # benchmark.variants.reuse-external-delta-parent = yes
  # benchmark.variants.revs                        = any-1-extra-rev
with-populate: 4.798632
no-populate:   4.953295   (+3.22%, +0.15)
  # benchmark.variants.revs                        = any-100-extra-rev
with-populate: 4.903618
no-populate:   5.014963   (+2.27%, +0.11)
 ## benchmark.name                                 = hg.command.unbundle
  # benchmark.variants.revs                        = any-1-extra-rev
with-populate: 1.423411
no-populate:   1.585365  (+11.38%, +0.16)
  # benchmark.variants.revs                        = any-100-extra-rev
with-populate: 1.537909
no-populate:   1.688489   (+9.79%, +0.15)

      hgrc settings    command line options      final result   
    quiet verbo debug   quiet verbo debug      quiet verbo debug
 0  False False False   False False False  ->  False False False
 1   True False False   False False False  ->   True False False
 2  False  True False   False False False  ->  False  True False
 3   True  True False   False False False  ->  False False False
 4  False False  True   False False False  ->  False  True  True
 5   True False  True   False False False  ->  False  True  True
 6  False  True  True   False False False  ->  False  True  True
 7   True  True  True   False False False  ->  False  True  True
 8  False False False    True False False  ->   True False False
 9   True False False    True False False  ->   True False False
10  False  True False    True False False  ->   True False False
11   True  True False    True False False  ->   True False False
12  False False  True    True False False  ->   True False False
13   True False  True    True False False  ->   True False False
14  False  True  True    True False False  ->   True False False
15   True  True  True    True False False  ->   True False False
16  False False False   False  True False  ->  False  True False
17   True False False   False  True False  ->  False  True False
18  False  True False   False  True False  ->  False  True False
19   True  True False   False  True False  ->  False  True False
20  False False  True   False  True False  ->  False  True False
21   True False  True   False  True False  ->  False  True False
22  False  True  True   False  True False  ->  False  True False
23   True  True  True   False  True False  ->  False  True False
24  False False False    True  True False  ->  False False False
25   True False False    True  True False  ->  False False False
26  False  True False    True  True False  ->  False False False
27   True  True False    True  True False  ->  False False False
28  False False  True    True  True False  ->  False False False
29   True False  True    True  True False  ->  False False False
30  False  True  True    True  True False  ->  False False False
31   True  True  True    True  True False  ->  False False False
32  False False False   False False  True  ->  False  True  True
33   True False False   False False  True  ->  False  True  True
34  False  True False   False False  True  ->  False  True  True
35   True  True False   False False  True  ->  False  True  True
36  False False  True   False False  True  ->  False  True  True
37   True False  True   False False  True  ->  False  True  True
38  False  True  True   False False  True  ->  False  True  True
39   True  True  True   False False  True  ->  False  True  True
40  False False False    True False  True  ->  False  True  True
41   True False False    True False  True  ->  False  True  True
42  False  True False    True False  True  ->  False  True  True
43   True  True False    True False  True  ->  False  True  True
44  False False  True    True False  True  ->  False  True  True
45   True False  True    True False  True  ->  False  True  True
46  False  True  True    True False  True  ->  False  True  True
47   True  True  True    True False  True  ->  False  True  True
48  False False False   False  True  True  ->  False  True  True
49   True False False   False  True  True  ->  False  True  True
50  False  True False   False  True  True  ->  False  True  True
51   True  True False   False  True  True  ->  False  True  True
52  False False  True   False  True  True  ->  False  True  True
53   True False  True   False  True  True  ->  False  True  True
54  False  True  True   False  True  True  ->  False  True  True
55   True  True  True   False  True  True  ->  False  True  True
56  False False False    True  True  True  ->  False  True  True
57   True False False    True  True  True  ->  False  True  True
58  False  True False    True  True  True  ->  False  True  True
59   True  True False    True  True  True  ->  False  True  True
60  False False  True    True  True  True  ->  False  True  True
61   True False  True    True  True  True  ->  False  True  True
62  False  True  True    True  True  True  ->  False  True  True
63   True  True  True    True  True  True  ->  False  True  True