problems with mmap() and disk caching
Alan Cox
alc at rice.edu
Fri Apr 6 09:06:59 UTC 2012
On 04/06/2012 03:38, Konstantin Belousov wrote:
> On Thu, Apr 05, 2012 at 01:25:49PM -0500, Alan Cox wrote:
>> On 04/05/2012 12:31, Konstantin Belousov wrote:
>>> On Thu, Apr 05, 2012 at 10:54:31AM -0500, 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.
>>> Yes, I agree that such change shall be an improvement, and I expect
>>> that Andrey will test it.
>>>
>>> On the other hand, I do think that allocator should prefer unnamed
>>> pages to pages which still have valid content. On my 12G desktop,
>>> I never saw more then 100MB of cached pages, and similar numbers
>>> are observed on the 32-48GB servers. I suppose that this is related.
>> On allocation, the system does prefer free pages over cached pages.
>> When cached pages are added to the physical memory allocator, they are
>> added to VM_FREEPOOL_CACHE. When pages are allocated, they are taken
>> from VM_FREEPOOL_DEFAULT. Generally, pages only move from the CACHE
>> pool to the DEFAULT pool when the DEFAULT pool is depleted. (However,
>> occasionally, they do move because of coalescing.) When I redid the
>> physical memory allocator, I looked at the rate of cached page
>> reactivation under the old and the new allocators. At least for the
>> tests that I did the rates weren't that different. It was low,
>> single-digit percentages. I think the highest likelihood of
>> reactivation comes from the pages that are cached by the sequential
>> access heuristic because it is so overzealous.
>>
>> I don't think it's related. You see modest numbers of cached pages
>> simply because the page daemon met its target for the sum of free and
>> cached pages. So, it just stopped moving pages from the inactive queue
>> into the physical memory allocator's cache/free queues.
> No, I mean something else. Specifically, I mean that somehow the
> preference for non-named pages does not work. At least, I cannot give
> any other explanation for the following experiment.
>
> Lets take stock HEAD without change in vm_fault.c. The initial
> state of 8GB machine is as follows, the test file was not even
> stat(2)-ed yet.
> Mem: 37M Active, 18M Inact, 150M Wired, 236K Cache, 27M Buf, 7612M Free
>
> Now, run the unmodified original Andrey' test with only one pass,
> making sequential read of the mmap of a 5GB file from UFS volume.
> After the run
> Mem: 38M Active, 18M Inact, 153M Wired, 21M Cache, 30M Buf, 7586M Free
>
> Please note that cached count increased only for 20M, and this is
> for calls to vm_page_cache() worth of 5GB. In other words, it seems
> that allocator almost never touches free memory, always preferring
> cache. This is mostly coincides with what I saw when I profiled
> original problem reported by Andrey.
Ah, I understand.
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