Re: [PATCH v3 0/2] sctp: delay calls to sk_data_ready() as much as possible

[David Miller <davem@davemloft.net>]

From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Date: Thu, 14 Apr 2016 14:00:49 -0300

> Em 14-04-2016 10:03, Neil Horman escreveu:
>> On Wed, Apr 13, 2016 at 11:05:32PM -0400, David Miller wrote:
>>> From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
>>> Date: Fri,  8 Apr 2016 16:41:26 -0300
>>>
>>>> 1st patch is a preparation for the 2nd. The idea is to not call
>>>> ->sk_data_ready() for every data chunk processed while processing
>>>> packets but only once before releasing the socket.
>>>>
>>>> v2: patchset re-checked, small changelog fixes
>>>> v3: on patch 2, make use of local vars to make it more readable
>>>
>>> Applied to net-next, but isn't this reduced overhead coming at the
>>> expense of latency?  What if that lower latency is important to the
>>> application and/or consumer?
>> Thats a fair point, but I'd make the counter argument that, as it
>> currently
>> stands, any latency introduced (or removed), is an artifact of our
>> implementation rather than a designed feature of it.  That is to say,
>> we make no
>> guarantees at the application level regarding how long it takes to
>> signal data
>> readines from the time we get data off the wire, so I would rather see
>> our
>> throughput raised if we can, as thats been sctp's more pressing
>> achilles heel.
>>
>>
>> Thats not to say I'd like to enable lower latency, but I'd rather have
>> this now,
>> and start pondering how to design that in.  Perhaps we can convert the
>> pending
>> flag to a counter to count the number of events we enqueue, and call
>> sk_data_ready every  time we reach a sysctl defined threshold.
> 
> That and also that there is no chance of the application reading the
> first chunks before all current ToDo's are performed by either the bh
> or backlog handlers for that packet. Socket lock won't be cycled in
> between chunks so the application is going to wait all the processing
> one way or another.

But it takes time to signal the wakeup to the remote cpu the process
was running on, schedule out the current process on that cpu (if it
has in fact lost it's timeslice), and then finally look at the socket
queue.

Of course this is all assuming the process was sleeping in the first
place, either in recv or more likely poll.

I really think signalling early helps performance.