linux-kernel - Re: zswap: How to determine whether it is compressing swap pages?

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Date:	Thu, 18 Jul 2013 20:43:40 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Martin Steigerwald <Martin@...htvoll.de>
CC:	Seth Jennings <sjenning@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: zswap: How to determine whether it is compressing swap pages?

Hi Martin,

On 07/18/2013 03:38 AM, Martin Steigerwald wrote:
> Am Mittwoch, 17. Juli 2013, 09:38:34 schrieb Seth Jennings:
>> On Wed, Jul 17, 2013 at 01:41:44PM +0200, Martin Steigerwald wrote:
>>> Is there any way to run zcache concurrently with zswap? I.e. use zcache only
>>> for read caches for filesystem and zswap for swap?
>>
>> No, at least not with zcache's frontswap features enabled.  frontswap is a very
>> simple API that allows only one "backend" to register with it at a time.  So
>> that means _either_ zswap or zcache.
>>
>> The only way they can be used in a meaningful way together is to use the
>> "nofrontswap" zcache option in the kernel boot parameters to prevent
>> zcache overriding zswap's frontswap registration.
>>
>> But the general answer is no, they shouldn't be used together.
>>
>>
>>>
>>> What is better suited for swap? zswap or zcache?
>>
>> zswap targets the specific case of caching swapped out pages in a compressed
>> cache and this is much simpler than zcache. zswap is also in mainline as of
>> 3.11-rc1.
> 
> Thanks.
> 
> Okay, then I will test zswap for now. I have a nice use case for it: Playing

Could you make some test by kernel compiling? Something like kernbench.
During my testing, I found that the swap ins/outs operations reduced but
the kernel compile time didn't reduce accordingly.

-- 
Regards,
-Bob
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