netdev - Re: Poorer networking performance in later kernels?

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Date:	Fri, 15 Apr 2016 15:37:21 -0700
From:	Rick Jones <rick.jones2@....com>
To:	"Butler, Peter" <pbutler@...usnet.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Poorer networking performance in later kernels?

On 04/15/2016 02:02 PM, Butler, Peter wrote:
> (Please keep me CC'd to all comments/responses)
>
> I've tried a kernel upgrade from 3.4.2 to 4.4.0 and see a marked drop
> in networking performance.  Nothing was changed on the test systems,
> other than the kernel itself (and kernel modules).  The identical
> .config used to build the 3.4.2 kernel was brought over into the
> 4.4.0 kernel source tree, and any configuration differences (e.g. new
> parameters, etc.) were taken as default values.
>
> The testing was performed on the same actual hardware for both kernel
> versions (i.e. take the existing 3.4.2 physical setup, simply boot
> into the (new) kernel and run the same test).  The netperf utility
> was used for benchmarking and the testing was always performed on
> idle systems.
>
> TCP testing yielded the following results, where the 4.4.0 kernel
> only got about 1/2 of the throughput:
>

>        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
>
> 3.4.2 13631488 13631488   8952    30.01      9370.29    10.14    6.50     0.709   0.454
> 4.4.0 13631488 13631488   8952    30.02      5314.03    9.14     14.31    1.127   1.765
>
> SCTP testing yielded the following results, where the 4.4.0 kernel only got about 1/3 of the throughput:
>
>        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
>
> 3.4.2 13631488 13631488   8952    30.00      2306.22    13.87    13.19    3.941   3.747
> 4.4.0 13631488 13631488   8952    30.01       882.74    16.86    19.14    12.516  14.210
>
> The same tests were performed a multitude of time, and are always
> consistent (within a few percent).  I've also tried playing with
> various run-time kernel parameters (/proc/sys/kernel/net/...) on the
> 4.4.0 kernel to alleviate the issue but have had no success at all.
>
> I'm at a loss as to what could possibly account for such a discrepancy...
>

I suspect I am not alone in being curious about the CPU(s) present in 
the systems and the model/whatnot of the NIC being used.  I'm also 
curious as to why you have what at first glance seem like absurdly large 
socket buffer sizes.

That said, it looks like you have some Really Big (tm) increases in 
service demand.  Many more CPU cycles being consumed per KB of data 
transferred.

Your message size makes me wonder if you were using a 9000 byte MTU.

Perhaps in the move from 3.4.2 to 4.4.0 you lost some or all of the 
stateless offloads for your NIC(s)?  Running ethtool -k <interface> on 
both ends under both kernels might be good.

Also, if you did have a 9000 byte MTU under 3.4.2 are you certain you 
still had it under 4.4.0?

It would (at least to me) also be interesting to run a TCP_RR test 
comparing the two kernels.  TCP_RR (at least with the default 
request/response size of one byte) doesn't really care about stateless 
offloads or MTUs and could show how much difference there is in basic 
path length (or I suppose in interrupt coalescing behaviour if the NIC 
in question has a mildly dodgy heuristic for such things).

happy benchmarking,

rick jones