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Date: Thu, 17 Dec 2009 14:28:10 -0800
From: Sridhar Samudrala <sri@...ibm.com>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Krishna Kumar2 <krkumar2@...ibm.com>,
"David S. Miller" <davem@...emloft.net>,
Jarek Poplawski <jarkao2@...il.com>, mst@...hat.com,
netdev@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC PATCH] Regression in linux 2.6.32 virtio_net seen with
vhost-net
On Thu, 2009-12-17 at 13:50 -0800, Sridhar Samudrala wrote:
> On Thu, 2009-12-17 at 22:36 +0800, Herbert Xu wrote:
> > On Thu, Dec 17, 2009 at 08:05:02PM +0530, Krishna Kumar2 wrote:
> > >
> > > I think the bug is in this check:
> > >
> > > + if (vi->capacity >= 2 + MAX_SKB_FRAGS) {
> > > + /* Suppress further xmit interrupts. */
> > > + vi->svq->vq_ops->disable_cb(vi->svq);
> > > + napi_complete(xmit_napi);
> > > +
> > > + /* Don't wake it if link is down. */
> > > + if (likely(netif_carrier_ok(vi->vdev)))
> > > + netif_wake_queue(vi->dev);
> > > + }
> > >
> > > We wake up too fast, just enough space for one more skb to be sent
> > > before the queue is stopped again. And hence no more messages about
> > > queue full, but lot of requeues. The qdisc code is doing the correct
> > > thing, but we need to increase the limit here.
> > >
> > > Can we try with some big number, 64, 128?
> >
> > Good point. Sridhar, could you please test doubling or quadrupling
> > the threshold? 128 is probably a bit too much though as the ring
> > only has 256 entries.
>
> Increasing the wakeup threshold value reduced the number of requeues, but
> didn't eliminate them. The throughput improved a little, but the CPU utilization
> went up.
> I don't see any 'queue full' warning messages from the driver and hence the driver
> is not returning NETDEV_TX_BUSY. The requeues are happening in sch_direct_xmit()
> as it is finding that the tx queue is stopped.
>
> I could not get 2.6.31 virtio-net driver to work with 2.6.32 kernel by simply
> replacing virtio-net.c. The compile and build went through fine, but the guest
> is not seeing the virtio-net device when it comes up.
> I think it is a driver issue, not a core issue as I am able to get good results
> by not stopping the queue early in start_xmit() and dropping the skb when xmit_skb()
> fails even with 2.6.32 kernel. I think this behavior is somewhat similar to 2.6.31
> virtio-net driver as it caches 1 skb and drops any further skbs when ring is full
> in its start_xmit routine.
Another interesting observation that Jarek pointed out is that if we drop skb's, i guess
it is causing TCP to backoff and enabling TCP GSO code to send much larger packets(64K
compared to 16K). So although the throughput is higher, packets/sec is less in that
case.
>
> 2.6.32 + Rusty's xmit_napi v2 patch
> -----------------------------------
> $ ./netperf -c -C -H 192.168.122.1 -t TCP_STREAM -l60
> TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.122.1 (192.168.122.1) port 0 AF_INET
> Recv Send Send Utilization Service Demand
> Socket Socket Message Elapsed Send Recv Send Recv
> Size Size Size Time Throughput local remote local remote
> bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB
>
> 87380 16384 16384 60.03 3255.22 87.16 82.57 2.193 4.156
> $ tc -s qdisc show dev eth0
> qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
> Sent 24524210050 bytes 1482737 pkt (dropped 0, overlimits 0 requeues 339101)
> rate 0bit 0pps backlog 0b 0p requeues 339101
>
> 2.6.32 + Rusty's xmit_napi v2 patch + wakeup threshold=64
> ---------------------------------------------------------
> $ ./netperf -c -C -H 192.168.122.1 -t TCP_STREAM -l60
> TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.122.1 (192.168.122.1) port 0 AF_INET
> Recv Send Send Utilization Service Demand
> Socket Socket Message Elapsed Send Recv Send Recv
> Size Size Size Time Throughput local remote local remote
> bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB
>
> 87380 16384 16384 60.03 3356.71 95.41 89.56 2.329 4.372
> [sridhar@...alhost ~]$ tc -s qdisc show dev eth0
> qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
> Sent 25290227186 bytes 1555119 pkt (dropped 0, overlimits 0 requeues 78179)
> rate 0bit 0pps backlog 0b 0p requeues 78179
>
> 2.6.32 + Rusty's xmit_napi v2 patch + wakeup threshold=128
> ----------------------------------------------------------
> $./netperf -c -C -H 192.168.122.1 -t TCP_STREAM -l60
> TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.122.1 (192.168.122.1) port 0 AF_INET
> Recv Send Send Utilization Service Demand
> Socket Socket Message Elapsed Send Recv Send Recv
> Size Size Size Time Throughput local remote local remote
> bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB
>
> 87380 16384 16384 60.03 3413.79 96.30 89.79 2.311 4.309
> [sridhar@...alhost ~]$ tc -s qdisc show dev eth0
> qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
> Sent 25719585472 bytes 1579448 pkt (dropped 0, overlimits 0 requeues 40299)
> rate 0bit 0pps backlog 0b 0p requeues 40299
>
> 2.6.32 + Rusty's xmit_napi v2 patch + don't stop early & drop skb on fail patch
> -------------------------------------------------------------------------------
> $./netperf -c -C -H 192.168.122.1 -t TCP_STREAM -l60
> TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 192.168.122.1 (192.168.122.1) port 0 AF_INET
> Recv Send Send Utilization Service Demand
> Socket Socket Message Elapsed Send Recv Send Recv
> Size Size Size Time Throughput local remote local remote
> bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB
>
> 87380 16384 16384 60.03 7741.65 70.09 72.84 0.742 1.542
> [sridhar@...alhost ~]$ tc -s qdisc show dev eth0
> qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
> Sent 58149531018 bytes 897991 pkt (dropped 0, overlimits 0 requeues 1)
> rate 0bit 0pps backlog 0b 0p requeues 1
>
>
>
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