problems with mmap() and disk caching
Alan Cox
alc at rice.edu
Tue Apr 10 16:27:10 UTC 2012
On 04/09/2012 10:26, John Baldwin wrote:
> 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?
>
Not yet. I did some tests with a file that was several times larger
than DRAM, and I didn't like what I saw. Initially, everything behaved
as expected, but about halfway through the test the bulk of the pages
were active. Despite the call to pmap_clear_reference() in
vm_page_dontneed(), the page daemon is finding the pages to be
referenced and reactivating them. The net result is that the time it
takes to read the file (from a relatively fast SSD) goes up by about
12%. So, this still needs work.
Alan
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