linux-kernel - Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

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Message-ID: <4BD52D55.3070803@redhat.com>
Date:	Mon, 26 Apr 2010 09:06:13 +0300
From:	Avi Kivity <avi@...hat.com>
To:	ngupta@...are.org
CC:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, jeremy@...p.org,
	hugh.dickins@...cali.co.uk, JBeulich@...ell.com,
	chris.mason@...cle.com, kurt.hackel@...cle.com,
	dave.mccracken@...cle.com, npiggin@...e.de,
	akpm@...ux-foundation.org, riel@...hat.com
Subject: Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

On 04/25/2010 07:05 PM, Nitin Gupta wrote:
>
>>> Increasing the frequency of discards is also not an option:
>>>    - Creating discard bio requests themselves need memory and these
>>> swap devices
>>> come into picture only under low memory conditions.
>>>
>>>        
>> That's fine, swap works under low memory conditions by using reserves.
>>
>>      
> Ok, but still all this bio allocation and block layer overhead seems
> unnecessary and is easily avoidable. I think frontswap code needs
> clean up but at least it avoids all this bio overhead.
>    

Ok.  I agree it is silly to go through the block layer and end up 
servicing it within the kernel.

>>>    - We need to regularly scan swap_map to issue these discards.
>>> Increasing discard
>>> frequency also means more frequent scanning (which will still not be
>>> fast enough
>>> for ramzswap needs).
>>>
>>>        
>> How does frontswap do this?  Does it maintain its own data structures?
>>
>>      
> frontswap simply calls frontswap_flush_page() in swap_entry_free() i.e. as
> soon as a swap slot is freed. No bio allocation etc.
>    

The same code could also issue the discard?

>> Even for copying to RAM an async API is wanted, so you can dma it
>> instead of copying.
>>
>>      
> Maybe incremental development is better? Stabilize and refine existing
> code and gradually move to async API, if required in future?
>    

Incremental development is fine, especially for ramzswap where the APIs 
are all internal.  I'm more worried about external interfaces, these 
stick around a lot longer and if not done right they're a pain forever.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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