sub milisec socket server write

Marc Lehmann schmorp at schmorp.de
Mon Feb 15 06:14:44 CET 2010


On Sun, Feb 14, 2010 at 05:25:10PM -0800, Neeraj Rai <rneeraj at yahoo.com> wrote:
> There is a trade off between latency and cpu usage. If we sleep 1ms after each queue drain, cpu usage is almost nil on no activity, but latency is ~1ms.
> If sleep is omitted, the cpu usage is 100% crowding out other procs on same box.
> Q1. is this the best way to do this ?

Depends - the best baseline is to use an event-based approach, and
if you need better latency, an extra real-time thread. In general,
sub-millisecond stuff can't really be done on a normal multiuser kernel
with high confidence - most kernels still work with time resolutions of
around 100hz.

Since you already have a thread for that socket, you should try switching
to blocking I/O: You already *pay* the thread overhead, so you might as
well let the kernel do the blocking for you.

> Q2. we tried registering for EV_WRITE but had same issue.

I have no clue what "same" refers to in your mail - what issue did you
get?

> Q3. ev_async seems to something that might help - each write to queue can 
>   call ev_async to wake the loop. However, before we knew about ev_async, 
>   we had attempted similar solution (write to pipe and register ev_READ on 
>   the pipe. This sometimes ran into deadlock when pipe was full. I think
>   pipes are 4K by default and we probably went over it.

Yeah, the dreaded netscape bug :)

>   Does the ev_async already take care of pipe full ?

ev_async doesn't necessarily use a pipe (it uses eventfd if available),
and if it uses one, it never writes more than a single octet.

Note that pipes are quite slow compared to memory manipulation (and
eventfd is even slower if your threads run on different cores, but that
should be avoided in general!).

So if you already pay for a thread and the thread switching, use it by
letting the thread read a queue and do blocking writes.

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