util: lower water mark when removing nodes after cost limit reached
See the inline comment for the reasoning here. This is a pretty
common strategy for garbage collectors, other cache-like primtives.
The performance impact is substantial:
$ hg perflrucachedict --size 4 --gets 1000000 --sets 1000000 --mixed 1000000 --costlimit 100
! inserts w/ cost limit
! wall 1.659181 comb 1.650000 user 1.650000 sys 0.000000 (best of 7)
! wall 1.722122 comb 1.720000 user 1.720000 sys 0.000000 (best of 6)
! mixed w/ cost limit
! wall 1.139955 comb 1.140000 user 1.140000 sys 0.000000 (best of 9)
! wall 1.182513 comb 1.180000 user 1.180000 sys 0.000000 (best of 9)
$ hg perflrucachedict --size 1000 --gets 1000000 --sets 1000000 --mixed 1000000 --costlimit 10000
! inserts
! wall 0.679546 comb 0.680000 user 0.680000 sys 0.000000 (best of 15)
! sets
! wall 0.825147 comb 0.830000 user 0.830000 sys 0.000000 (best of 13)
! inserts w/ cost limit
! wall 25.105273 comb 25.080000 user 25.080000 sys 0.000000 (best of 3)
! wall 1.724397 comb 1.720000 user 1.720000 sys 0.000000 (best of 6)
! mixed
! wall 0.807096 comb 0.810000 user 0.810000 sys 0.000000 (best of 13)
! mixed w/ cost limit
! wall 12.104470 comb 12.070000 user 12.070000 sys 0.000000 (best of 3)
! wall 1.190563 comb 1.190000 user 1.190000 sys 0.000000 (best of 9)
$ hg perflrucachedict --size 1000 --gets 1000000 --sets 1000000 --mixed 1000000 --costlimit 10000 --mixedgetfreq 90
! inserts
! wall 0.711177 comb 0.710000 user 0.710000 sys 0.000000 (best of 14)
! sets
! wall 0.846992 comb 0.850000 user 0.850000 sys 0.000000 (best of 12)
! inserts w/ cost limit
! wall 25.963028 comb 25.960000 user 25.960000 sys 0.000000 (best of 3)
! wall 2.184311 comb 2.180000 user 2.180000 sys 0.000000 (best of 5)
! mixed
! wall 0.728256 comb 0.730000 user 0.730000 sys 0.000000 (best of 14)
! mixed w/ cost limit
! wall 3.174256 comb 3.170000 user 3.170000 sys 0.000000 (best of 4)
! wall 0.773186 comb 0.770000 user 0.770000 sys 0.000000 (best of 13)
$ hg perflrucachedict --size 100000 --gets 1000000 --sets 1000000 --mixed 1000000 --mixedgetfreq 90 --costlimit 5000000
! gets
! wall 1.191368 comb 1.190000 user 1.190000 sys 0.000000 (best of 9)
! wall 1.195304 comb 1.190000 user 1.190000 sys 0.000000 (best of 9)
! inserts
! wall 0.950995 comb 0.950000 user 0.950000 sys 0.000000 (best of 11)
! inserts w/ cost limit
! wall 1.589732 comb 1.590000 user 1.590000 sys 0.000000 (best of 7)
! sets
! wall 1.094941 comb 1.100000 user 1.090000 sys 0.010000 (best of 9)
! mixed
! wall 0.936420 comb 0.940000 user 0.930000 sys 0.010000 (best of 10)
! mixed w/ cost limit
! wall 0.882780 comb 0.870000 user 0.870000 sys 0.000000 (best of 11)
This puts us ~2x slower than caches without cost accounting. And for
read-heavy workloads (the prime use cases for caches), performance is
nearly identical.
In the worst case (pure write workloads with cost accounting enabled),
we're looking at ~1.5us per insert on large caches. That seems "fast
enough."
Differential Revision: https://phab.mercurial-scm.org/D4505
A new repository uses zlib storage, which doesn't need a requirement
$ hg init default
$ cd default
$ cat .hg/requires
dotencode
fncache
generaldelta
revlogv1
store
testonly-simplestore (reposimplestore !)
$ touch foo
$ hg -q commit -A -m 'initial commit with a lot of repeated repeated repeated text to trigger compression'
$ hg debugrevlog -c | grep 0x78
0x78 (x) : 1 (100.00%)
0x78 (x) : 110 (100.00%)
$ cd ..
Unknown compression engine to format.compression aborts
$ hg --config experimental.format.compression=unknown init unknown
abort: compression engine unknown defined by experimental.format.compression not available
(run "hg debuginstall" to list available compression engines)
[255]
A requirement specifying an unknown compression engine results in bail
$ hg init unknownrequirement
$ cd unknownrequirement
$ echo exp-compression-unknown >> .hg/requires
$ hg log
abort: repository requires features unknown to this Mercurial: exp-compression-unknown!
(see https://mercurial-scm.org/wiki/MissingRequirement for more information)
[255]
$ cd ..
#if zstd
$ hg --config experimental.format.compression=zstd init zstd
$ cd zstd
$ cat .hg/requires
dotencode
exp-compression-zstd
fncache
generaldelta
revlogv1
store
testonly-simplestore (reposimplestore !)
$ touch foo
$ hg -q commit -A -m 'initial commit with a lot of repeated repeated repeated text'
$ hg debugrevlog -c | grep 0x28
0x28 : 1 (100.00%)
0x28 : 98 (100.00%)
$ cd ..
Specifying a new format.compression on an existing repo won't introduce data
with that engine or a requirement
$ cd default
$ touch bar
$ hg --config experimental.format.compression=zstd -q commit -A -m 'add bar with a lot of repeated repeated repeated text'
$ cat .hg/requires
dotencode
fncache
generaldelta
revlogv1
store
testonly-simplestore (reposimplestore !)
$ hg debugrevlog -c | grep 0x78
0x78 (x) : 2 (100.00%)
0x78 (x) : 199 (100.00%)
#endif