Messages in this thread | | | Date | Mon, 13 Apr 2015 14:38:25 +0100 | From | Jonathan Davies <> | Subject | Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance regression on Xen |
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On 13/04/15 11:56, George Dunlap wrote: > On Thu, Apr 9, 2015 at 5:36 PM, Stefano Stabellini > <stefano.stabellini@eu.citrix.com> wrote: >> On Thu, 9 Apr 2015, Eric Dumazet wrote: >>> On Thu, 2015-04-09 at 16:46 +0100, Stefano Stabellini wrote: >>>> Hi all, >>>> >>>> I found a performance regression when running netperf -t TCP_MAERTS from >>>> an external host to a Xen VM on ARM64: v3.19 and v4.0-rc4 running in the >>>> virtual machine are 30% slower than v3.18. >>>> >>>> Through bisection I found that the perf regression is caused by the >>>> prensence of the following commit in the guest kernel: >>>> >>>> >>>> commit 605ad7f184b60cfaacbc038aa6c55ee68dee3c89 >>>> Author: Eric Dumazet <edumazet@google.com> >>>> Date: Sun Dec 7 12:22:18 2014 -0800 >>>> >>>> tcp: refine TSO autosizing > > [snip]
I recently discussed this issue on netdev in the following thread:
https://www.marc.info/?l=linux-netdev&m=142738853820517
>>> This commit restored original TCP Small Queue behavior, which is the >>> first step to fight bufferbloat. >>> >>> Some network drivers are known to be problematic because of a delayed TX >>> completion. > > [snip] > >>> Try to tweak /proc/sys/net/ipv4/tcp_limit_output_bytes to see if it >>> makes a difference ? >> >> A very big difference: >> >> echo 262144 > /proc/sys/net/ipv4/tcp_limit_output_bytes >> brings us much closer to the original performance, the slowdown is just >> 8% >> >> echo 1048576 > /proc/sys/net/ipv4/tcp_limit_output_bytes >> fills the gap entirely, same performance as before "refine TSO >> autosizing" >> >> >> What would be the next step for here? Should I just document this as an >> important performance tweaking step for Xen, or is there something else >> we can do? > > Is the problem perhaps that netback/netfront delays TX completion? > Would it be better to see if that can be addressed properly, so that > the original purpose of the patch (fighting bufferbloat) can be > achieved while not degrading performance for Xen? Or at least, so > that people get decent perfomance out of the box without having to > tweak TCP parameters?
I agree; reducing the completion latency should be the ultimate goal. However, that won't be easy, so we need a work-around in the short term. I don't like the idea of relying on documenting the recommendation to change tcp_limit_output_bytes; too many people won't read this advice and will expect the out-of-the-box defaults to be reasonable.
Following Eric's pointers to where a similar problem had been experienced in wifi drivers, I came up with two proof-of-concept patches that gave a similar performance gain without any changes to sysctl parameters or core tcp/ip code. See https://www.marc.info/?l=linux-netdev&m=142746161307283.
I haven't yet received any feedback from the xen-netfront maintainers about whether those ideas could be reasonably adopted.
Jonathan
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