Re: [PATCH RFC] mm/swap: automatic tuning for swapin readahead

[Hugh Dickins]

On Mon, 22 Oct 2012, Shaohua Li wrote:
> 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);

Thank you for playing and trying that, I haven't found time to revisit it
at all.  I'll give that adjustment a go at my end.  The "+ 1" was for the
target page itself; but whatever works best, there's not much science to it.

> 
> 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?

But only in the simplest cases is the sequentiality of placement on swap
well correlated with the sequentiality of placement in virtual memory.
Once you have a sequential access thread and a random access thread
swapping out at the same time, their pages will be interspersed.

I'm pretty sure that if you give it more thought than I am giving it
at the moment, you can devise a test case which would go amazingly
faster by your per-vma method than by keeping just this global state.

But I doubt such a test case would be so realistic as to deserve that
extra sophistication.  I do prefer to keep the heuristic as stupid and
unpretentious as possible.

Especially when I remember how get_swap_page() stripes across swap
areas of equal priority: my guess is that nobody uses that feature,
and we don't even want to consider it here; but it feels wrong to
ignore it if we aim for more cleverness at the readahead end.

Hugh
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