netdev - Re: [PATCH net-next 0/2] net: broadcom: Adaptive interrupt coalescing

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Date:   Mon, 26 Mar 2018 16:40:23 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Tal Gilboa <talgi@...lanox.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, jaedon.shin@...il.com, pgynther@...gle.com,
        opendmb@...il.com, michael.chan@...adcom.com, gospo@...adcom.com,
        saeedm@...lanox.com
Subject: Re: [PATCH net-next 0/2] net: broadcom: Adaptive interrupt coalescing

On 03/26/2018 04:21 PM, Tal Gilboa wrote:
> On 3/27/2018 1:29 AM, Florian Fainelli wrote:
>> On 03/26/2018 03:04 PM, Florian Fainelli wrote:
>>> On 03/26/2018 02:16 PM, Tal Gilboa wrote:
>>>> On 3/23/2018 4:19 AM, Florian Fainelli wrote:
>>>>> Hi all,
>>>>>
>>>>> This patch series adds adaptive interrupt coalescing for the Gigabit
>>>>> Ethernet
>>>>> drivers SYSTEMPORT and GENET.
>>>>>
>>>>> This really helps lower the interrupt count and system load, as
>>>>> measured by
>>>>> vmstat for a Gigabit TCP RX session:
>>>>
>>>> I don't see an improvement in system load, the opposite - 42% vs. 100%
>>>> for SYSTEMPORT and 85% vs. 100% for GENET. Both with the same
>>>> bandwidth.
>>>
>>> Looks like I did not extract the correct data the load could spike in
>>> both cases (with and without net_dim) up to 100, but averaged over the
>>> transmission I see the following:
>>>
>>> GENET without:
>>>   1  0      0 1169568      0  25556    0    0     0     0 130079
>>> 62795  2
>>> 86 13  0  0
>>>
>>> GENET with:
>>>   1  0      0 1169536      0  25556    0    0     0     0 10566 10869  1
>>> 21 78  0  0
>>>
>>>> Am I missing something? Talking about bandwidth, I would expect 941Mb/s
>>>> (assuming this is TCP over IPv4). Do you know why the reduced interrupt
>>>> rate doesn't improve bandwidth?
>>>
>>> I am assuming that this comes down to a latency, still capturing some
>>> pcap files to analyze the TCP session with wireshark and see if that is
>>> indeed what is going on. The test machine is actually not that great
> 
> I would expect 1GbE full wire speed on almost any setup. I'll try
> applying your code on my setup and see what I get.

The test machine that I am using appears to be loaded by other non
networking workload which perturbs the tests I am running, other than
than I agree, wire speed should be expected.

> 
>>>
>>>> Also, any effect on the client side (you
>>>> mentioned enabling TX moderation for SYSTEMPORT)?
>>>
>>> Yes, on SYSTEMPORT, being the TCP IPv4 client, I have the following:
>>>
>>> SYSTEMPORT without:
>>>   2  0      0 191428      0  25748    0    0     0     0 86254  264 
>>> 0 41
>>> 59  0  0
>>>
>>> SYSTEMPORT with:
>>>   3  0      0 190176      0  25748    0    0     0     0 45485 31332  0
>>> 100  0  0  0
>>>
>>> I don't get top to agree with these load results though but it looks
>>> like we just have the CPU spinning more, does not look like a win.
>>
>> The problem appears to be the timeout selection on TX, ignoring it
>> completely allows us to keep the load average down while maintaining the
>> bandwidth. Looks like NAPI on TX already does a good job, so interrupt
>> mitigation on TX is not such a great idea actually...
> 
> I saw a similar behavior for TX. For me the issue was too many
> outstanding bytes without a completion (defined to be 256KB by sysctl
> net.ipv4.tcp_limit_output_bytes). I tested on a 100GbE connection so
> with reasonable timeout values I already waited too long (4 TSO
> sessions). For the 1GbE case this might have no effect since you need a
> very long timeout. I'm currently working on adding TX support for dim.
> If you don't see a good benefit currently you might want to wait a
> little with TX adaptive interrupt moderation. Maybe only adjust static
> moderation for now?

Yes static moderation appears to be doing just fine.

> 
>>
>> Also, doing UDP TX tests shows that we can lower the interrupt count by
>> setting an appropriate tx-frames (as expected), but we won't be lowering
>> the CPU load since that is inherently a CPU intensive work. Past
> 
> Do you see higher TX UDP bandwidth? If you are bounded by CPU on both
> cases I would at least expect higher bandwidth with less interrupts
> since you reduce work from the CPU.

UDP bandwidth was intentionally limited the UDP bandwidth to
800Mbits/sec, we are definitively not CPU bound (18% CPU load), but we
can still lower the interrupt count.
-- 
Florian