linux-kernel - Re: [PATCH v2 2/2] virtio-ring: Allocate indirect buffers from cache when possible

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Date:	Mon, 10 Sep 2012 11:01:06 -0500
From:	Thomas Lendacky <tahm@...ux.vnet.ibm.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Sasha Levin <levinsasha928@...il.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, kvm@...r.kernel.org
Subject: Re: [PATCH v2 2/2] virtio-ring: Allocate indirect buffers from cache when possible

On Friday, September 07, 2012 09:19:04 AM Rusty Russell wrote:
> "Michael S. Tsirkin" <mst@...hat.com> writes:
> > On Thu, Sep 06, 2012 at 05:27:23PM +0930, Rusty Russell wrote:
> >> "Michael S. Tsirkin" <mst@...hat.com> writes:
> >> > Yes without checksum net core always linearizes packets, so yes it is
> >> > screwed.
> >> > For -net, skb always allocates space for 17 frags + linear part so
> >> > it seems sane to do same in virtio core, and allocate, for -net,
> >> > up to max_frags + 1 from cache.
> >> > We can adjust it: no _SG -> 2 otherwise 18.
> >> 
> >> But I thought it used individual buffers these days?
> > 
> > Yes for receive, no for transmit. That's probably why
> > we should have the threshold per vq, not per device, BTW.
> 
> Can someone actually run with my histogram patch and see what the real
> numbers are?

Somehow some HTML got in my first reply, resending...

I ran some TCP_RR and TCP_STREAM sessions, both host-to-guest and
guest-to-host, with a form of the histogram patch applied against a
RHEL6.3 kernel. The histogram values were reset after each test.

Here are the results:

60 session TCP_RR from host-to-guest with 256 byte request and 256 byte
response for 60 seconds:

Queue histogram for virtio1:
Size distribution for input (max=7818456):
 1: 7818456 ################################################################
Size distribution for output (max=7816698):
 2: 149     
 3: 7816698 ################################################################
 4: 2       
 5: 1       
Size distribution for control (max=1):
 0: 0


4 session TCP_STREAM from host-to-guest with 4K message size for 60 seconds:

Queue histogram for virtio1:
Size distribution for input (max=16050941):
 1: 16050941 ################################################################
Size distribution for output (max=1877796):
 2: 1877796 ################################################################
 3: 5       
Size distribution for control (max=1):
 0: 0

4 session TCP_STREAM from host-to-guest with 16K message size for 60 seconds:

Queue histogram for virtio1:
Size distribution for input (max=16831151):
 1: 16831151 ################################################################
Size distribution for output (max=1923965):
 2: 1923965 ################################################################
 3: 5       
Size distribution for control (max=1):
 0: 0

4 session TCP_STREAM from guest-to-host with 4K message size for 60 seconds:

Queue histogram for virtio1:
Size distribution for input (max=1316069):
 1: 1316069 ################################################################
Size distribution for output (max=879213):
 2: 24      
 3: 24097   #
 4: 23176   #
 5: 3412    
 6: 4446    
 7: 4663    
 8: 4195    
 9: 3772    
10: 3388    
11: 3666    
12: 2885    
13: 2759    
14: 2997    
15: 3060    
16: 2651    
17: 2235    
18: 92721   ######  
19: 879213  ################################################################
Size distribution for control (max=1):
 0: 0

4 session TCP_STREAM from guest-to-host with 16K message size for 60 seconds:

Queue histogram for virtio1:
Size distribution for input (max=1428590):
 1: 1428590 ################################################################
Size distribution for output (max=957774):
 2: 20
 3: 54955   ###
 4: 34281   ##
 5: 2967
 6: 3394
 7: 9400
 8: 3061
 9: 3397
10: 3258
11: 3275
12: 3147
13: 2876
14: 2747
15: 2832
16: 2013
17: 1670
18: 100369  ######
19: 957774  ################################################################
Size distribution for control (max=1):
 0: 0

Thanks,
Tom

> 
> I'm not convinced that the ideal 17-buffer case actually happens as much
> as we think.  And if it's not happening with this netperf test, we're
> testing the wrong thing.
> 
> Thanks,
> Rusty.
> 
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