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

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Date:	Thu, 22 Apr 2010 19:13:08 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, jeremy@...p.org,
	hugh.dickins@...cali.co.uk, ngupta@...are.org, 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 04/22/2010 06:48 PM, Dan Magenheimer wrote:
>>> a synchronous concurrency-safe page-oriented pseudo-RAM device (such
>>>   :
>>> conform to certain policies as follows:
>>>        
>> How baked in is the synchronous requirement?  Memory, for example, can
>> be asynchronous if it is copied by a dma engine, and since there are
>> hardware encryption engines, there may be hardware compression engines
>> in the future.
>>      
> Thanks for the comment!
>
> Synchronous is required, but likely could be simulated by ensuring all
> coherency (and concurrency) requirements are met by some intermediate
> "buffering driver" -- at the cost of an extra page copy into a buffer
> and overhead of tracking the handles (poolid/inode/index) of pages in
> the buffer that are "in flight".  This is an approach we are considering
> to implement an SSD backend, but hasn't been tested yet so, ahem, the
> proof will be in the put'ing. ;-)
>    

Well, copying memory so you can use a zero-copy dma engine is 
counterproductive.

Much easier to simulate an asynchronous API with a synchronous backend.

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

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