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

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Date:	Mon, 26 Apr 2010 19:17:02 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Avi Kivity <avi@...hat.com>
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 04/26/2010 11:36 AM, Avi Kivity wrote:
> 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?
> 


No, we cannot issue discard bio at this place since swap_lock
spinlock is held.


Thanks,
Nitin
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