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Re: [Xen-devel] [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

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.


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