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Date: Wed, 3 Oct 2012 14:07:41 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Konstantin Khlebnikov <khlebnikov@...nvz.org>
cc: Shaohua Li <shli@...nel.org>, Rik van Riel <riel@...hat.com>,
Minchan Kim <minchan@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Wu Fengguang <fengguang.wu@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH RFC] mm/swap: automatic tuning for swapin readahead
On Tue, 2 Oct 2012, Konstantin Khlebnikov wrote:
> Hugh Dickins wrote:
> >
> > If I boot with mem=900M (and 1G swap: either on hard disk sda, or
> > on Vertex II SSD sdb), and mmap anonymous 1000M (either MAP_PRIVATE,
> > or MAP_SHARED for a shmem object), and either cycle sequentially round
> > that making 5M touches (spaced a page apart), or make 5M random touches,
> > then here are the times in centisecs that I see (but it's only elapsed
> > that I've been worrying about).
> >
> > 3.6-rc7 swapping to hard disk:
> > 124 user 6154 system 73921 elapsed -rc7 sda seq
> > 102 user 8862 system 895392 elapsed -rc7 sda random
> > 130 user 6628 system 73601 elapsed -rc7 sda shmem seq
> > 194 user 8610 system 1058375 elapsed -rc7 sda shmem random
> >
> > 3.6-rc7 swapping to SSD:
> > 116 user 5898 system 24634 elapsed -rc7 sdb seq
> > 96 user 8166 system 43014 elapsed -rc7 sdb random
> > 110 user 6410 system 24959 elapsed -rc7 sdb shmem seq
> > 208 user 8024 system 45349 elapsed -rc7 sdb shmem random
> >
> > 3.6-rc7 + Shaohua's patch (and FAULT_FLAG_RETRY check in do_swap_page),
> > HDD:
> > 116 user 6258 system 76210 elapsed shli sda seq
> > 80 user 7716 system 831243 elapsed shli sda random
> > 128 user 6640 system 73176 elapsed shli sda shmem seq
> > 212 user 8522 system 1053486 elapsed shli sda shmem random
> >
> > 3.6-rc7 + Shaohua's patch (and FAULT_FLAG_RETRY check in do_swap_page),
> > SSD:
> > 126 user 5734 system 24198 elapsed shli sdb seq
> > 90 user 7356 system 26146 elapsed shli sdb random
> > 128 user 6396 system 24932 elapsed shli sdb shmem seq
> > 192 user 8006 system 45215 elapsed shli sdb shmem random
> >
> > 3.6-rc7 + my patch, swapping to hard disk:
> > 126 user 6252 system 75611 elapsed hugh sda seq
> > 70 user 8310 system 871569 elapsed hugh sda random
> > 130 user 6790 system 73855 elapsed hugh sda shmem seq
> > 148 user 7734 system 827935 elapsed hugh sda shmem random
> >
> > 3.6-rc7 + my patch, swapping to SSD:
> > 116 user 5996 system 24673 elapsed hugh sdb seq
> > 76 user 7568 system 28075 elapsed hugh sdb random
> > 132 user 6468 system 25052 elapsed hugh sdb shmem seq
> > 166 user 7220 system 28249 elapsed hugh sdb shmem random
> >
>
> Hmm, It would be nice to gather numbers without swapin readahead at all, just
> to see the the worst possible case for sequential read and the best for
> random.
Right, and also interesting to see what happens if we raise page_cluster
(more of an option than it was, with your or my patch scaling it down).
Run on the same machine under the same conditions:
3.6-rc7 + my patch, swapping to hard disk with page_cluster 0 (no readahead):
136 user 34038 system 121542 elapsed hugh cluster0 sda seq
102 user 7928 system 841680 elapsed hugh cluster0 sda random
130 user 34770 system 118322 elapsed hugh cluster0 sda shmem seq
160 user 7362 system 756489 elapsed hugh cluster0 sda shmem random
3.6-rc7 + my patch, swapping to SSD with page_cluster 0 (no readahead):
138 user 32230 system 70018 elapsed hugh cluster0 sdb seq
88 user 7296 system 25901 elapsed hugh cluster0 sdb random
154 user 33150 system 69678 elapsed hugh cluster0 sdb shmem seq
166 user 6936 system 24332 elapsed hugh cluster0 sdb shmem random
3.6-rc7 + my patch, swapping to hard disk with page_cluster 4 (default + 1):
144 user 4262 system 77950 elapsed hugh cluster4 sda seq
74 user 8268 system 863871 elapsed hugh cluster4 sda random
140 user 4880 system 73534 elapsed hugh cluster4 sda shmem seq
160 user 7788 system 834804 elapsed hugh cluster4 sda shmem random
3.6-rc7 + my patch, swapping to SSD with page_cluster 4 (default + 1):
124 user 4242 system 21125 elapsed hugh cluster4 sdb seq
72 user 7680 system 28686 elapsed hugh cluster4 sdb random
122 user 4622 system 21387 elapsed hugh cluster4 sdb shmem seq
172 user 7238 system 28226 elapsed hugh cluster4 sdb shmem random
I was at first surprised to see random significantly faster than sequential
on SSD with readahead off, thinking they ought to come out the same. But
no, that's a warning on the limitations of the test: with an mmap of 1000M
on a machine with mem=900M, the page-by-page sequential is never going to
rehit cache, whereas the random has a good chance of finding in memory.
Which I presume also accounts for the lower user times throughout
for random - but then why not the same for shmem random?
I did start off measuring on the laptop with SSD, mmap 1000M mem=500M;
but once I transferred to the desktop, I rediscovered just how slow
swapping to hard disk can be, couldn't wait days, so made mem=900M.
> I'll run some tests too, especially I want to see how it works for less
> synthetic workloads.
Thank you, that would be valuable. I expect there to be certain midway
tests on which Shaohao's patch would show up as significantly faster,
where his per-vma approach would beat the global approach; then the
global to improve with growing contention between processes. But I
didn't devise any such test, and hoped Shaohua might have one.
Hugh
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