Re: [PATCH 2/3] vmscan: make mapped executable pages the firstclass citizen
From: Wu Fengguang
Date: Tue May 19 2009 - 03:53:45 EST
On Tue, May 19, 2009 at 03:20:19PM +0800, KOSAKI Motohiro wrote:
> > On Tue, May 19, 2009 at 12:41:38PM +0800, KOSAKI Motohiro wrote:
> > > Hi
> > >
> > > Thanks for great works.
> > >
> > >
> > > > SUMMARY
> > > > =======
> > > > The patch decreases the number of major faults from 50 to 3 during 10% cache hot reads.
> > > >
> > > >
> > > > SCENARIO
> > > > ========
> > > > The test scenario is to do 100000 pread(size=110 pages, offset=(i*100) pages),
> > > > where 10% of the pages will be activated:
> > > >
> > > > for i in `seq 0 100 10000000`; do echo $i 110; done > pattern-hot-10
> > > > iotrace.rb --load pattern-hot-10 --play /b/sparse
> > >
> > >
> > > Which can I download iotrace.rb?
> > >
> > >
> > > > and monitor /proc/vmstat during the time. The test box has 2G memory.
> > > >
> > > >
> > > > ANALYZES
> > > > ========
> > > >
> > > > I carried out two runs on fresh booted console mode 2.6.29 with the VM_EXEC
> > > > patch, and fetched the vmstat numbers on
> > > >
> > > > (1) begin: shortly after the big read IO starts;
> > > > (2) end: just before the big read IO stops;
> > > > (3) restore: the big read IO stops and the zsh working set restored
> > > >
> > > > nr_mapped nr_active_file nr_inactive_file pgmajfault pgdeactivate pgfree
> > > > begin: 2481 2237 8694 630 0 574299
> > > > end: 275 231976 233914 633 776271 20933042
> > > > restore: 370 232154 234524 691 777183 20958453
> > > >
> > > > begin: 2434 2237 8493 629 0 574195
> > > > end: 284 231970 233536 632 771918 20896129
> > > > restore: 399 232218 234789 690 774526 20957909
> > > >
> > > > and another run on 2.6.30-rc4-mm with the VM_EXEC logic disabled:
> > >
> > > I don't think it is proper comparision.
> > > you need either following comparision. otherwise we insert many guess into the analysis.
> > >
> > > - 2.6.29 with and without VM_EXEC patch
> > > - 2.6.30-rc4-mm with and without VM_EXEC patch
> > >
> > >
> > > >
> > > > begin: 2479 2344 9659 210 0 579643
> > > > end: 284 232010 234142 260 772776 20917184
> > > > restore: 379 232159 234371 301 774888 20967849
> > > >
> > > > The numbers show that
> > > >
> > > > - The startup pgmajfault of 2.6.30-rc4-mm is merely 1/3 that of 2.6.29.
> > > > I'd attribute that improvement to the mmap readahead improvements :-)
> > > >
> > > > - The pgmajfault increment during the file copy is 633-630=3 vs 260-210=50.
> > > > That's a huge improvement - which means with the VM_EXEC protection logic,
> > > > active mmap pages is pretty safe even under partially cache hot streaming IO.
> > > >
> > > > - when active:inactive file lru size reaches 1:1, their scan rates is 1:20.8
> > > > under 10% cache hot IO. (computed with formula Dpgdeactivate:Dpgfree)
> > > > That roughly means the active mmap pages get 20.8 more chances to get
> > > > re-referenced to stay in memory.
> > > >
> > > > - The absolute nr_mapped drops considerably to 1/9 during the big IO, and the
> > > > dropped pages are mostly inactive ones. The patch has almost no impact in
> > > > this aspect, that means it won't unnecessarily increase memory pressure.
> > > > (In contrast, your 20% mmap protection ratio will keep them all, and
> > > > therefore eliminate the extra 41 major faults to restore working set
> > > > of zsh etc.)
> >
> > More results on X desktop, kernel 2.6.30-rc4-mm:
> >
> > nr_mapped nr_active_file nr_inactive_file pgmajfault pgdeactivate pgfree
> >
> > VM_EXEC protection ON:
> > begin: 9740 8920 64075 561 0 678360
> > end: 768 218254 220029 565 798953 21057006
> > restore: 857 218543 220987 606 799462 21075710
> > restore X: 2414 218560 225344 797 799462 21080795
> >
> > VM_EXEC protection OFF:
> > begin: 9368 5035 26389 554 0 633391
> > end: 770 218449 221230 661 646472 17832500
> > restore: 1113 218466 220978 710 649881 17905235
> > restore X: 2687 218650 225484 947 802700 21083584
> >
> > The added "restore X" means after IO, switch back and forth between the urxvt
> > and firefox windows to restore their working set. I cannot explain why the
> > absolute nr_mapped grows larger at the end of VM_EXEC OFF case. Maybe it's
> > because urxvt is the foreground window during the first run, and firefox is the
> > foreground window during the second run?
> >
> > Like the console mode, the absolute nr_mapped drops considerably - to 1/13 of
> > the original size - during the streaming IO.
> >
> > The delta of pgmajfault is 3 vs 107 during IO, or 236 vs 393 during the whole
> > process.
>
> hmmm.
>
> about 100 page fault don't match Elladan's problem, I think.
> perhaps We missed any addional reproduce condition?
Elladan's case is not the point of this test.
Elladan's IO is use-once, so probably not a caching problem at all.
This test case is specifically devised to confirm whether this patch
works as expected. Conclusion: it is.
Thanks,
Fengguang
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