netdev - Re: [PATCH net-next] Revert "xen-netback: Aggregate TX unmap operations"

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Date:	Wed, 26 Mar 2014 10:57:02 +0000
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	<wei.liu2@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<paul.durrant@...rix.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <jonathan.davies@...rix.com>
Subject: Re: [PATCH net-next] Revert "xen-netback: Aggregate TX unmap operations"

On 24/03/14 09:26, Ian Campbell wrote:
> On Fri, 2014-03-21 at 17:23 +0000, Zoltan Kiss wrote:
>> This reverts commit e9275f5e2df1b2098a8cc405d87b88b9affd73e6. This commit is the
>> last in the netback grant mapping series, and it tries to do more aggressive
>> aggreagtion of unmap operations. However practical use showed almost no
>> positive effect, whilst with certain frontends it causes significant performance
>> regression.
>
> That's a shame -- do you have any insight into why?

It cause performance regression when the guest limits itself to a small 
amount of outstanding packets. E.g. with iperf on Win7 there are always 
2 in flight.
Currently batching happens in this way:
- the callback can put up to MAX_SKB_FRAGS slots into the dealloc ring 
before it wakes up the dealloc thread
- the thread doesn't schedule immediately, of course, so other callbacks 
can add to the dealloc ring in the meantime
- and even when the dealloc thread consumes the dealloc ring, the 
callbacks can put slots onto it

And my upcoming patch will avoid TLB flush in a lot of cases. If someone 
has more time to research a better strategy, that would be good, but I 
think currently it is a low priority thing.

Zoli
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