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SubjectRe: [PATCH 0/8] zcache: page cache compression support
On 07/20/2010 07:58 PM, Dan Magenheimer wrote:
>> On 07/20/2010 01:27 AM, Dan Magenheimer wrote:
>>>> We only keep pages that compress to PAGE_SIZE/2 or less. Compressed
>>>> chunks are
>>>> stored using xvmalloc memory allocator which is already being used
>> by
>>>> zram
>>>> driver for the same purpose. Zero-filled pages are checked and no
>>>> memory is
>>>> allocated for them.
>>>
>>> I'm curious about this policy choice. I can see why one
>>> would want to ensure that the average page is compressed
>>> to less than PAGE_SIZE/2, and preferably PAGE_SIZE/2
>>> minus the overhead of the data structures necessary to
>>> track the page. And I see that this makes no difference
>>> when the reclamation algorithm is random (as it is for
>>> now). But once there is some better reclamation logic,
>>> I'd hope that this compression factor restriction would
>>> be lifted and replaced with something much higher. IIRC,
>>> compression is much more expensive than decompression
>>> so there's no CPU-overhead argument here either,
>>> correct?
>>
>> Its true that we waste CPU cycles for every incompressible page
>> encountered but still we can't keep such pages in RAM since this
>> is what host wanted to reclaim and we can't help since compression
>> failed. Compressed caching makes sense only when we keep highly
>> compressible pages in RAM, regardless of reclaim scheme.
>>
>> Keeping (nearly) incompressible pages in RAM probably makes sense
>> for Xen's case where cleancache provider runs *inside* a VM, sending
>> pages to host. So, if VM is limited to say 512M and host has 64G RAM,
>> caching guest pages, with or without compression, will help.
>
> I agree that the use model is a bit different, but PAGE_SIZE/2
> still seems like an unnecessarily strict threshold. For
> example, saving 3000 clean pages in 2000*PAGE_SIZE of RAM
> still seems like a considerable space savings. And as
> long as the _average_ is less than some threshold, saving
> a few slightly-less-than-ideally-compressible pages doesn't
> seem like it would be a problem. For example, IMHO, saving two
> pages when one compresses to 2047 bytes and the other compresses
> to 2049 bytes seems just as reasonable as saving two pages that
> both compress to 2048 bytes.
>
> Maybe the best solution is to make the threshold a sysfs
> settable? Or maybe BOTH the single-page threshold and
> the average threshold as two different sysfs settables?
> E.g. throw away a put page if either it compresses poorly
> or adding it to the pool would push the average over.
>

Considering overall compression average instead of bothering about
individual page compressibility seems like a good point. Still, I think
storing completely incompressible pages isn't desirable.

So, I agree with the idea of separate sysfs tunables for average and single-page
compression thresholds with defaults conservatively set to 50% and PAGE_SIZE/2
respectively. I will include these in "v2" patches.

Thanks,
Nitin



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