linux-kernel - Re: [PATCH v3 2/2] virtio-ring: Allocate indirect buffers from cache when possible

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Date:	Tue, 04 Sep 2012 19:34:19 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Sasha Levin <levinsasha928@...il.com>, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 2/2] virtio-ring: Allocate indirect buffers from cache
 when possible

On 08/31/2012 12:56 PM, Michael S. Tsirkin wrote:
> On Fri, Aug 31, 2012 at 11:36:07AM +0200, Sasha Levin wrote:
>> On 08/30/2012 03:38 PM, Michael S. Tsirkin wrote:
>> >> +static unsigned int indirect_alloc_thresh = 16;
>> > Why 16?  Please make is MAX_SG + 1 this makes some sense.
>> 
>> Wouldn't MAX_SG mean we always allocate from the cache? Isn't the memory waste
>> too big in this case?
> 
> Sorry. I really meant MAX_SKB_FRAGS + 1. MAX_SKB_FRAGS is 17 so gets us
> threshold of 18. It is less than the size of an skb+shinfo itself so -
> does it look too big to you? Also why do you think 16 is not too big but
> 18 is?  If there's a reason then I am fine with 16 too but then please
> put it in code comment near where the value is set.
> 
> Yes this means virtio net always allocates from cache
> but this is a good thing, isn't it? Gets us more consistent
> performance.

kmalloc() also goes to a cache.  Is there a measurable difference?

Ugh, there's an ugly loop in __find_general_cachep(), which really wants
to be replaced with fls().

-- 
error compiling committee.c: too many arguments to function
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