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

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Date:	Sat, 24 Apr 2010 21:27:46 +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 04/24/2010 04:49 AM, Nitin Gupta wrote:
>
>> I see.  So why not implement this as an ordinary swap device, with a
>> higher priority than the disk device?  this way we reuse an API and keep
>> things asynchronous, instead of introducing a special purpose API.
>>
>>      
> ramzswap is exactly this: an ordinary swap device which stores every page
> in (compressed) memory and its enabled as highest priority swap. Currently,
> it stores these compressed chunks in guest memory itself but it is not very
> difficult to send these chunks out to host/hypervisor using virtio.
>
> However, it suffers from unnecessary block I/O layer overhead and requires
> weird hooks in swap code, say to get notification when a swap slot is freed.
>    

Isn't that TRIM?

> OTOH frontswap approach gets rid of any such artifacts and overheads.
> (ramzswap: http://code.google.com/p/compcache/)
>    

Maybe we should optimize these overheads instead.  Swap used to always 
be to slow devices, but swap-to-flash has the potential to make swap act 
like an extension of RAM.

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

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