Date: Mon, 19 Aug 2013 23:33:04 +0400 From: Vitja Makarov <vitja.makarov@gmail.com> To: freebsd-current@freebsd.org Subject: Question about socket timeouts Message-ID: <CAKGHGPS=HCYfXxPXuUz5G83j5sGieejPU-QHmi9TrmMhmweHLw@mail.gmail.com>
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Hi!
Recently I was playing with small socket timeouts. setsockopt(2)
SO_RCVTIMEO and found a problem with it: if timeout is small enough
read(2) may return before timeout is actually expired.
I was unable to reproduce this on linux box.
I found that kernel uses a timer with 1/HZ precision so it converts
time in microseconds to ticks that's ok linux does it as well. The
problem is in details: freebsd uses floor() approach while linux uses
ceil():
from FreeBSD's sys/kern/uipc_socket.c:
val = (u_long)(tv.tv_sec * hz) + tv.tv_usec / tick;
if (val == 0 && tv.tv_usec != 0)
val = 1; /* at least one tick if tv > 0 */
from Linux's net/core/sock.c:
*timeo_p = tv.tv_sec*HZ + (tv.tv_usec+(1000000/HZ-1))/(1000000/HZ);
So, for instance, we have a freebsd system running with kern.hz set
100 and set receive timeout to 25ms that is converted to 2 ticks which
is 20ms. In my test program read(2) returns with EAGAIN set in
0.019ms.
So the question is: is that a problem or not?
--
vitja.
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