From: "Tan, Jianfeng" <jianfeng.tan@intel.com>
To: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Cc: dev@dpdk.org, zhihong.wang@intel.com
Subject: Re: [dpdk-dev] [PATCH] examples/vhost: fix perf regression
Date: Wed, 20 Jul 2016 10:44:13 +0800 [thread overview]
Message-ID: <baea10f5-e0ae-e644-7bb0-41a5a81cbe9a@intel.com> (raw)
In-Reply-To: <20160720014450.GQ5146@yliu-dev.sh.intel.com>
Hi Yuanhan,
On 7/20/2016 9:44 AM, Yuanhan Liu wrote:
> On Tue, Jul 19, 2016 at 01:53:11PM +0000, Jianfeng Tan wrote:
>> We find significant perfermance drop introduced by below commit,
>> when vhost example is started with --mergeable 0 and inside vm,
>> kernel virtio-net driver is used to do ip based forwarding.
>>
>> The root cause is that below commit adds support for
>> VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6, and when
>> mergeable is disabled, it triggers big_packets path of virtio-net
>> driver. In this path, virtio driver uses 19 desc with 18 4K-sized
>> pages to receive each packet, so that it can receive a big packet
>> with size of 64K. But QEMU only creates 256 desc entries for each
>> vq, which results in that only 13 packets can be received. VM
>> kernel can quickly handle those packets and go to sleep (HLT).
>>
>> As QEMU has no option to set the desc entries of a vq, so here,
>> we disable VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6
>> with VIRTIO_NET_F_HOST_TSO4 and VIRTIO_NET_F_HOST_TSO6 when we
>> disable tso of vhost example, to avoid VM kernel virtio driver
>> go into big_packets path.
>>
>> Fixes: 859b480d5afd ("vhost: add guest offload setting")
>>
>> Reported-by: Qian Xu <qian.q.xu@intel.com>
>> Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
> We could apply this patch, but I don't think it actually fix anything:
>
> - it doesn't fix other vhost applications, say OVS, which is for sure
> way more widly used than vhost-example.
If I remember it correctly, OVS will enable mergeable.
>
> - it doesn't even fix it when tso is enabled and mergeable-rx is disabled
> with this vhost-example.
But we'd better avoid users go into such doubt that performance drops
because of that commit under the case tso=off,mergeable=off, right?
Thanks,
Jianfeng
>
> Thanks for the good root-cause, btw!
>
> --yliu
next prev parent reply other threads:[~2016-07-20 2:44 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-19 13:53 Jianfeng Tan
2016-07-20 1:44 ` Yuanhan Liu
2016-07-20 2:44 ` Tan, Jianfeng [this message]
2016-07-20 3:16 ` Xu, Qian Q
2016-07-20 4:00 ` Yuanhan Liu
2016-07-20 3:52 ` Xu, Qian Q
2016-07-20 4:38 ` Yuanhan Liu
2016-07-20 5:50 ` Tan, Jianfeng
2016-07-20 6:13 ` Yuanhan Liu
2016-07-20 6:30 ` Tan, Jianfeng
2016-07-21 0:23 ` [dpdk-dev] [PATCH v2] " Jianfeng Tan
2016-07-21 0:42 ` Tan, Jianfeng
2016-07-21 0:42 ` [dpdk-dev] [PATCH v3] " Jianfeng Tan
2016-07-21 1:34 ` Yuanhan Liu
2016-07-21 1:38 ` Xu, Qian Q
2016-07-22 9:59 ` Thomas Monjalon
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=baea10f5-e0ae-e644-7bb0-41a5a81cbe9a@intel.com \
--to=jianfeng.tan@intel.com \
--cc=dev@dpdk.org \
--cc=yuanhan.liu@linux.intel.com \
--cc=zhihong.wang@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).