linux-kernel - Re: [PATCH RFC 06/12] xen-blkback: implement LRU mechanism for persistent grants

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Message-ID: <51474838.2070206@citrix.com>
Date:	Mon, 18 Mar 2013 18:00:40 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: Re: [PATCH RFC 06/12] xen-blkback: implement LRU mechanism for persistent
 grants

On 05/03/13 22:49, Konrad Rzeszutek Wilk wrote:
>>> This could be written a bit differently to also run outside the xen_blkif_schedule
>>> (so a new thread). This would require using the lock mechanism and converting
>>> this big loop to two smaller loops:
>>>  1) - one quick that holds the lock - to take the items of the list,
>>>  2) second one to do the grant_set_unmap_op operations and all the heavy
>>>     free_xenballooned_pages call.
>>
>> Yes, I could add a list_head to persistent_gnt, so we can take them out
>> of the red-black tree and queue them in a list to be processed (unmap +
>> free) after we have looped thought the list, without holding the lock.

I've been trying to implement the "purge" on a different kthread, but
I'm not able to get the same performance. Since moving this a different
thread requires additional contention (spinlocks) around the red-black
tree of persistent grants, I think we should leave it as-is right now,
and consider moving it to a different thread if we can get a performance
benefit.
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