From: Yuanhan Liu <yuanhan.liu@linux.intel.com>
To: Kevin Traynor <ktraynor@redhat.com>
Cc: maxime.coquelin@redhat.com, dev@dpdk.org, stable@dpdk.org
Subject: Re: [dpdk-stable] [PATCH] vhost: fix virtio_net cache sharing of broadcast_rarp
Date: Thu, 16 Mar 2017 14:21:22 +0800 [thread overview]
Message-ID: <20170316062122.GN18844@yliu-dev.sh.intel.com> (raw)
In-Reply-To: <1489605049-18686-1-git-send-email-ktraynor@redhat.com>
On Wed, Mar 15, 2017 at 07:10:49PM +0000, Kevin Traynor wrote:
> The virtio_net structure is used in both enqueue and dequeue datapaths.
> broadcast_rarp is checked with cmpset in the dequeue datapath regardless
> of whether descriptors are available or not.
>
> It is observed in some cases where dequeue and enqueue are performed by
> different cores and no packets are available on the dequeue datapath
> (i.e. uni-directional traffic), the frequent checking of broadcast_rarp
> in dequeue causes performance degradation for the enqueue datapath.
>
> In OVS the issue can cause a uni-directional performance drop of up to 15%.
>
> Fix that by moving broadcast_rarp to a different cache line in
> virtio_net struct.
Thanks, but I'm a bit confused. The drop looks like being caused by
cache false sharing, but I don't see anything would lead to a false
sharing. I mean, there is no write in the same cache line where the
broadcast_rarp belongs. Or, the "volatile" type is the culprit here?
Talking about that, I had actually considered to turn "broadcast_rarp"
to a simple "int" or "uint16_t" type, to make it more light weight.
The reason I used atomic type is to exactly send one broadcast RARP
packet once SEND_RARP request is recieved. Otherwise, we may send more
than one RARP packet when MQ is invovled. But I think we don't have
to be that accurate: it's tolerable when more RARP are sent. I saw 4
SEND_RARP requests (aka 4 RARP packets) in the last time I tried
vhost-user live migration after all. I don't quite remember why
it was 4 though.
That said, I think it also would resolve the performance issue if you
change "rte_atomic16_t" to "uint16_t", without moving the place?
--yliu
next prev parent reply other threads:[~2017-03-16 6:23 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-15 19:10 Kevin Traynor
2017-03-16 6:21 ` Yuanhan Liu [this message]
2017-03-16 10:10 ` Kevin Traynor
2017-03-17 5:47 ` Yuanhan Liu
2017-03-17 10:01 ` Maxime Coquelin
2017-03-20 11:13 ` Kevin Traynor
2017-03-23 15:44 ` [dpdk-stable] [PATCH v2] vhost: fix virtio_net false sharing Kevin Traynor
2017-03-27 7:34 ` Maxime Coquelin
2017-03-27 8:33 ` Yuanhan Liu
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