Re: [PATCH RFC] mm/swap: automatic tuning for swapin readahead
From: Shaohua Li
Date: Mon Oct 22 2012 - 03:37:01 EST
On Tue, Oct 16, 2012 at 08:50:49AM +0800, Shaohua Li wrote:
> On Mon, Oct 08, 2012 at 03:09:58PM -0700, 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.
> > Did you draw any conclusions from the numbers you found?
> > 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.
> Ok, I tested Hugh's patch. My test is a multithread random write workload.
> With Hugh's patch, 49:28.06elapsed
> With mine, 43:23.39elapsed
> There is 12% more time used with Hugh's patch.
> In the stable state of this workload, SI:SO ratio should be roughly 1:1. With
> Hugh's patch, it's around 1.6:1, there is still unnecessary swapin.
> I also tried a workload with seqential/random write mixed, Hugh's patch is 10%
> bad too.
With below change, the si/so ratio is back to around 1:1 in my workload. Guess
the run time of my test will be reduced too, though I didn't test yet.
- used = atomic_xchg(&swapra_hits, 0) + 1;
+ used = atomic_xchg(&swapra_hits, 0);
I'm wondering how could a global counter based method detect readahead
correctly. For example, if there are a sequential access thread and a random
access thread, doesn't this method always make wrong decision?
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