Hugh Dickins wrote:
On Thu, 4 Oct 2012, Konstantin Khlebnikov wrote:
Here results of my test. Workload isn't very realistic, but at least it
threaded: compiling linux-3.6 with defconfig in 16 threads on tmpfs,
512mb ram, dualcore cpu, ordinary hard disk. (test script in attachment)
average results for ten runs:
RA=3 RA=0 RA=1 RA=2 RA=4 Hugh Shaohua
real time 500 542 528 519 500 523 522
user time 738 737 735 737 739 737 739
sys time 93 93 91 92 96 92 93
pgmajfault 62918 110533 92454 78221 54342 86601 77229
pgpgin 2070372 795228 1034046 1471010 3177192 1154532 1599388
pgpgout 2597278 2022037 2110020 2350380 2802670 2286671 2526570
pswpin 462747 138873 202148 310969 739431 232710 341320
pswpout 646363 502599 524613 584731 697797 568784 628677
So, last two columns shows mostly equal results: +4.6% and +4.4% in
comparison to vanilla kernel with RA=3, but your version shows more stable
results (std-error 2.7% against 4.8%) (all this numbers in huge table in
attachment)
Thanks for doing this, Konstantin, but I'm stuck for anything much to say!
Shaohua and I are both about 4.5% bad for this particular test, but I'm
more consistently bad - hurrah!
I suspect (not a convincing argument) that if the test were just slightly
different (a little more or a little less memory, SSD instead of hard
disk, diskcache instead of tmpfs), then it would come out differently.
Yes, results depends mostly on tmpfs.
Did you draw any conclusions from the numbers you found?
Yeah, I have some ideas:
Numbers for vanilla kernel shows strong dependence between time and readahead
size. Seems like main problem is that tmpfs does not have it's own readahead,
it can only rely on swap-in readahead. There are about 25% readahead hits for RA=3.
As "pswpin" row shows both your and Shaohua patches makes readahead smaller.
Plus tmpfs doesn't keeps copy for clean pages in the swap (unlike to anon pages).
On swapin path it always marks page dirty and releases swap-entry.
I didn't have any measurements but this particular test definitely re-reads
some files multiple times and writes them back to the swap after that.
I haven't done any more on this in the last few days, except to verify
that once an anon_vma is judged random with Shaohua's, then it appears
to be condemned to no-readahead ever after.
That's probably something that a hack like I had in mine would fix,
but that addition might change its balance further (and increase vma
or anon_vma size) - not tried yet.
All I want to do right now, is suggest to Andrew that he hold Shaohua's
patch back from 3.7 for the moment: I'll send a response to Sep 7th's
mm-commits mail to suggest that - but no great disaster if he ignores me.
Hugh
Numbers from your tests formatted into table for better readability
HDD Vanilla Shaohua RA=3 RA=0 RA=4
SEQ, ANON 73921 76210 75611 121542 77950
SEQ, SHMEM 73601 73176 73855 118322 73534
RND, ANON 895392 831243 871569 841680 863871
RND, SHMEM 1058375 1053486 827935 756489 834804
SDD Vanilla Shaohua RA=3 RA=0 RA=4
SEQ, ANON 24634 24198 24673 70018 21125
SEQ, SHMEM 24959 24932 25052 69678 21387
RND, ANON 43014 26146 28075 25901 28686
RND, SHMEM 45349 45215 28249 24332 28226
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