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

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Message-ID: <50462DF5.1000806@redhat.com>
Date:	Tue, 04 Sep 2012 19:36:05 +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 09/04/2012 07:34 PM, Avi Kivity wrote:
> 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().
> 

Actually, not, as the loop will be very short for small sizes.

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