Subject: Re: [PATCH net-next v2] tcp: reduce cpu usage under tcp memory pressure when SO_SNDBUF is set

From: Jason Baron <[email-masked]>
Date: Fri, 21 Aug 2015 16:55:30 -0400

On 08/11/2015 01:59 PM, Jason Baron wrote:
> 
> 
> On 08/11/2015 12:12 PM, Eric Dumazet wrote:
>> On Tue, 2015-08-11 at 11:03 -0400, Jason Baron wrote:
>>
>>>
>>> Yes, so the test case I'm using to test against is somewhat contrived.
>>> In that I am simply allocating around 40,000 sockets that are idle to
>>> create a 'permanent' memory pressure in the background. Then, I have
>>> just 1 flow that sets SO_SNDBUF, which results in the: poll(), write() loop.
>>>
>>> That said, we encountered this issue initially where we had 10,000+
>>> flows and whenever the system would get into memory pressure, we would
>>> see all the cpus spin at 100%.
>>>
>>> So the testcase I wrote, was just a simplistic version for testing. But
>>> I am going to try and test against the more realistic workload where
>>> this issue was initially observed.
>>>
>>
>> Note that I am still trying to understand why we need to increase socket
>> structure, for something which is inherently a problem of sharing memory
>> with an unknown (potentially big) number of sockets.
>>
> 
> I was trying to mirror the wakeups when SO_SNDBUF is not set, where we
> continue to trigger on 1/3 of the buffer being available, as the
> sk->sndbuf is shrunk. And I saw this value as dynamic depending on
> number of sockets and read/write buffer usage. So that's where I was
> coming from with it.
> 
> Also, at least with the .config I have the tcp_sock structure didn't
> increase in size (although struct sock did go up by 8 and not 4).
> 
>> I suggested to use a flag (one bit).
>>
>> If set, then we should fallback to tcp_wmem[0] (each socket has 4096
>> bytes, so that we can avoid starvation)
>>
>>
>>
> 
> Ok, I will test this approach.

Hi Eric,

So I created a test here with 20,000 streams, and if I set SO_SNDBUF
high enough on the server side, I can create tcp memory pressure above
tcp_mem[2]. In this case, with the 'one bit' approach using tcp_wmem[0]
as the wakeup threshold I can still observe the 100% cpu spinning issue,
but with this v2 patch, cpu usage is minimal (1-2%). Since, we don't
guarantee tcp_wmem[0], above tcp_mem[2]. So using the 'one bit'
definitely alleviates the spinning between tcp_mem[1] and tcp_mem[2],
but not above tcp_mem[2] in my testing.

Maybe nobody cares about this case (you are getting what you ask for by
using SO_SNDBUF), but it seems to me that it would be nice to avoid this
sort of behavior. I also like the fact that with the
sk_effective_sndbuf, we keep doing wakeups on 1/3 of the write buffer
emptying, which keeps the wakeup behavior consistent. In theory this
would matter for high latency and bandwidth link, but in the testing I
did, I didn't observe any throughput differences between this v2 patch,
and the 'one bit' approach.

As I mentioned with this v2, the 'struct sock' grows by 4 bytes, but
struct tcp_sock does not increase. So since this is tcp specific, we
could add the sk_effective_sndbuf only to the struct tcp_sock.

So the 'one bit' approach definitely seems to me to be an improvement,
but I wanted to get feedback on this testing, before deciding how to
proceed.

Thanks,

-Jason