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Date:      Mon, 9 Apr 2012 11:26:25 -0400
From:      John Baldwin <jhb@freebsd.org>
To:        freebsd-hackers@freebsd.org
Cc:        Konstantin Belousov <kostikbel@gmail.com>, alc@freebsd.org, Andrey Zonov <andrey@zonov.org>, Alan Cox <alc@rice.edu>
Subject:   Re: problems with mmap() and disk caching
Message-ID:  <201204091126.25260.jhb@freebsd.org>
In-Reply-To: <4F7DC037.9060803@rice.edu>
References:  <4F7B495D.3010402@zonov.org> <20120404071746.GJ2358@deviant.kiev.zoral.com.ua> <4F7DC037.9060803@rice.edu>

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On Thursday, April 05, 2012 11:54:31 am Alan Cox wrote:
> On 04/04/2012 02:17, Konstantin Belousov wrote:
> > On Tue, Apr 03, 2012 at 11:02:53PM +0400, Andrey Zonov wrote:
> >> Hi,
> >>
> >> I open the file, then call mmap() on the whole file and get pointer,
> >> then I work with this pointer.  I expect that page should be only once
> >> touched to get it into the memory (disk cache?), but this doesn't work!
> >>
> >> I wrote the test (attached) and ran it for the 1G file generated from
> >> /dev/random, the result is the following:
> >>
> >> Prepare file:
> >> # swapoff -a
> >> # newfs /dev/ada0b
> >> # mount /dev/ada0b /mnt
> >> # dd if=/dev/random of=/mnt/random-1024 bs=1m count=1024
> >>
> >> Purge cache:
> >> # umount /mnt
> >> # mount /dev/ada0b /mnt
> >>
> >> Run test:
> >> $ ./mmap /mnt/random-1024 30
> >> mmap:  1 pass took:   7.431046 (none: 262112; res:     32; super:
> >> 0; other:      0)
> >> mmap:  2 pass took:   7.356670 (none: 261648; res:    496; super:
> >> 0; other:      0)
> >> mmap:  3 pass took:   7.307094 (none: 260521; res:   1623; super:
> >> 0; other:      0)
> >> mmap:  4 pass took:   7.350239 (none: 258904; res:   3240; super:
> >> 0; other:      0)
> >> mmap:  5 pass took:   7.392480 (none: 257286; res:   4858; super:
> >> 0; other:      0)
> >> mmap:  6 pass took:   7.292069 (none: 255584; res:   6560; super:
> >> 0; other:      0)
> >> mmap:  7 pass took:   7.048980 (none: 251142; res:  11002; super:
> >> 0; other:      0)
> >> mmap:  8 pass took:   6.899387 (none: 247584; res:  14560; super:
> >> 0; other:      0)
> >> mmap:  9 pass took:   7.190579 (none: 242992; res:  19152; super:
> >> 0; other:      0)
> >> mmap: 10 pass took:   6.915482 (none: 239308; res:  22836; super:
> >> 0; other:      0)
> >> mmap: 11 pass took:   6.565909 (none: 232835; res:  29309; super:
> >> 0; other:      0)
> >> mmap: 12 pass took:   6.423945 (none: 226160; res:  35984; super:
> >> 0; other:      0)
> >> mmap: 13 pass took:   6.315385 (none: 208555; res:  53589; super:
> >> 0; other:      0)
> >> mmap: 14 pass took:   6.760780 (none: 192805; res:  69339; super:
> >> 0; other:      0)
> >> mmap: 15 pass took:   5.721513 (none: 174497; res:  87647; super:
> >> 0; other:      0)
> >> mmap: 16 pass took:   5.004424 (none: 155938; res: 106206; super:
> >> 0; other:      0)
> >> mmap: 17 pass took:   4.224926 (none: 135639; res: 126505; super:
> >> 0; other:      0)
> >> mmap: 18 pass took:   3.749608 (none: 117952; res: 144192; super:
> >> 0; other:      0)
> >> mmap: 19 pass took:   3.398084 (none:  99066; res: 163078; super:
> >> 0; other:      0)
> >> mmap: 20 pass took:   3.029557 (none:  74994; res: 187150; super:
> >> 0; other:      0)
> >> mmap: 21 pass took:   2.379430 (none:  55231; res: 206913; super:
> >> 0; other:      0)
> >> mmap: 22 pass took:   2.046521 (none:  40786; res: 221358; super:
> >> 0; other:      0)
> >> mmap: 23 pass took:   1.152797 (none:  30311; res: 231833; super:
> >> 0; other:      0)
> >> mmap: 24 pass took:   0.972617 (none:  16196; res: 245948; super:
> >> 0; other:      0)
> >> mmap: 25 pass took:   0.577515 (none:   8286; res: 253858; super:
> >> 0; other:      0)
> >> mmap: 26 pass took:   0.380738 (none:   3712; res: 258432; super:
> >> 0; other:      0)
> >> mmap: 27 pass took:   0.253583 (none:   1193; res: 260951; super:
> >> 0; other:      0)
> >> mmap: 28 pass took:   0.157508 (none:      0; res: 262144; super:
> >> 0; other:      0)
> >> mmap: 29 pass took:   0.156169 (none:      0; res: 262144; super:
> >> 0; other:      0)
> >> mmap: 30 pass took:   0.156550 (none:      0; res: 262144; super:
> >> 0; other:      0)
> >>
> >> If I ran this:
> >> $ cat /mnt/random-1024>  /dev/null
> >> before test, when result is the following:
> >>
> >> $ ./mmap /mnt/random-1024 5
> >> mmap:  1 pass took:   0.337657 (none:      0; res: 262144; super:
> >> 0; other:      0)
> >> mmap:  2 pass took:   0.186137 (none:      0; res: 262144; super:
> >> 0; other:      0)
> >> mmap:  3 pass took:   0.186132 (none:      0; res: 262144; super:
> >> 0; other:      0)
> >> mmap:  4 pass took:   0.186535 (none:      0; res: 262144; super:
> >> 0; other:      0)
> >> mmap:  5 pass took:   0.190353 (none:      0; res: 262144; super:
> >> 0; other:      0)
> >>
> >> This is what I expect.  But why this doesn't work without reading file
> >> manually?
> > Issue seems to be in some change of the behaviour of the reserv or
> > phys allocator. I Cc:ed Alan.
> 
> I'm pretty sure that the behavior here hasn't significantly changed in 
> about twelve years.  Otherwise, I agree with your analysis.
> 
> On more than one occasion, I've been tempted to change:
> 
>                                          pmap_remove_all(mt);
>                                          if (mt->dirty != 0)
>                                                  vm_page_deactivate(mt);
>                                          else
>                                                  vm_page_cache(mt);
> 
> to:
> 
>                                          vm_page_dontneed(mt);
> 
> because I suspect that the current code does more harm than good.  In 
> theory, it saves activations of the page daemon.  However, more often 
> than not, I suspect that we are spending more on page reactivations than 
> we are saving on page daemon activations.  The sequential access 
> detection heuristic is just too easily triggered.  For example, I've 
> seen it triggered by demand paging of the gcc text segment.  Also, I 
> think that pmap_remove_all() and especially vm_page_cache() are too 
> severe for a detection heuristic that is so easily triggered.

Are you planning to commit this?

-- 
John Baldwin



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