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Date:	Tue, 2 Dec 2014 05:08:35 -0500 (EST)
From:	Pankaj Gupta <pagupta@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>
Cc:	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH RFC v4 net-next 0/5] virtio_net: enabling tx interrupts


> 
> On Tue, Dec 02, 2014 at 09:59:48AM +0008, Jason Wang wrote:
> > 
> > 
> > On Tue, Dec 2, 2014 at 5:43 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > >On Tue, Dec 02, 2014 at 08:15:02AM +0008, Jason Wang wrote:
> > >>     On Tue, Dec 2, 2014 at 11:15 AM, Jason Wang <jasowang@...hat.com>
> > >>wrote:
> > >> >
> > >> >
> > >> >On Mon, Dec 1, 2014 at 6:42 PM, Michael S. Tsirkin <mst@...hat.com>
> > >>wrote:
> > >> >>On Mon, Dec 01, 2014 at 06:17:03PM +0800, Jason Wang wrote:
> > >> >>> Hello:
> > >> >>>  We used to orphan packets before transmission for virtio-net. This
> > >> >>>breaks
> > >> >>> socket accounting and can lead serveral functions won't work, e.g:
> > >> >>>  - Byte Queue Limit depends on tx completion nofication to work.
> > >> >>> - Packet Generator depends on tx completion nofication for the last
> > >> >>>   transmitted packet to complete.
> > >> >>> - TCP Small Queue depends on proper accounting of sk_wmem_alloc to
> > >> >>>work.
> > >> >>>  This series tries to solve the issue by enabling tx interrupts. To
> > >> >>>minize
> > >> >>> the performance impacts of this, several optimizations were used:
> > >> >>>  - In guest side, virtqueue_enable_cb_delayed() was used to delay
> > >>the
> > >> >>>tx
> > >> >>>   interrupt untile 3/4 pending packets were sent.
> > >> >>> - In host side, interrupt coalescing were used to reduce tx
> > >> >>>interrupts.
> > >> >>>  Performance test results[1] (tx-frames 16 tx-usecs 16) shows:
> > >> >>>  - For guest receiving. No obvious regression on throughput were
> > >> >>>   noticed. More cpu utilization were noticed in few cases.
> > >> >>> - For guest transmission. Very huge improvement on througput for
> > >> >>>small
> > >> >>>   packet transmission were noticed. This is expected since TSQ and
> > >> >>>other
> > >> >>>   optimization for small packet transmission work after tx
> > >>interrupt.
> > >> >>>But
> > >> >>>   will use more cpu for large packets.
> > >> >>> - For TCP_RR, regression (10% on transaction rate and cpu
> > >> >>>utilization) were
> > >> >>>   found. Tx interrupt won't help but cause overhead in this case.
> > >> >>>Using
> > >> >>>   more aggressive coalescing parameters may help to reduce the
> > >> >>>regression.
> > >> >>
> > >> >>OK, you do have posted coalescing patches - does it help any?
> > >> >
> > >> >Helps a lot.
> > >> >
> > >> >For RX, it saves about 5% - 10% cpu. (reduce 60%-90% tx intrs)
> > >> >For small packet TX, it increases 33% - 245% throughput. (reduce about
> > >>60%
> > >> >inters)
> > >> >For TCP_RR, it increase the 3%-10% trans.rate. (reduce 40%-80% tx
> > >>intrs)
> > >> >
> > >> >>
> > >> >>I'm not sure the regression is due to interrupts.
> > >> >>It would make sense for CPU but why would it
> > >> >>hurt transaction rate?
> > >> >
> > >> >Anyway guest need to take some cycles to handle tx interrupts.
> > >> >And transaction rate does increase if we coalesces more tx interurpts.
> > >> >>
> > >> >>
> > >> >>It's possible that we are deferring kicks too much due to BQL.
> > >> >>
> > >> >>As an experiment: do we get any of it back if we do
> > >> >>-        if (kick || netif_xmit_stopped(txq))
> > >> >>-                virtqueue_kick(sq->vq);
> > >> >>+        virtqueue_kick(sq->vq);
> > >> >>?
> > >> >
> > >> >
> > >> >I will try, but during TCP_RR, at most 1 packets were pending,
> > >> >I suspect if BQL can help in this case.
> > >> Looks like this helps a lot in multiple sessions of TCP_RR.
> > >
> > >so what's faster
> > >	BQL + kick each packet
> > >	no BQL
> > >?
> > 
> > Quick and manual tests (TCP_RR 64, TCP_STREAM 512) does not show obvious
> > differences.
> > 
> > May need a complete benchmark to see.
> 
> Okay so going forward something like BQL + kick each packet
> might be a good solution.
> The advantage of BQL is that it works without GSO.
> For example, now that we don't do UFO, you might
> see significant gains with UDP.

If I understand correctly, it can also help for small packet
regr. in multiqueue scenario? Would be nice to see the perf. numbers
with multi-queue for small packets streams.
> 
> 
> > >
> > >
> > >> How about move the BQL patch out of this series?
> > >> Let's first converge tx interrupt and then introduce it?
> > >> (e.g with kicking after queuing X bytes?)
> > >
> > >Sounds good.
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