[Python-Dev] Modules/socketmodule.c: avoiding second fcntl() call worth the effort? (original) (raw)

Peter Portante peter.a.portante at gmail.com
Sun Jan 20 05:49:04 CET 2013

Hello folks,

I noticed while stracing a process that sock.setblocking() calls always result in pairs of fcntl() calls on Linux. Checking 2.6.8, 2.7.3, and 3.3.0 Modules/socketmodule.c, the code seems to use the following (unless I have missed something):

delay_flag = fcntl(s->sock_fd, F_GETFL, 0);
if (block)
    delay_flag &= (~O_NONBLOCK);
else
    delay_flag |= O_NONBLOCK;
fcntl(s->sock_fd, F_SETFL, delay_flag);

Perhaps a check to see the flags changed might be worth making?

int orig_delay_flag = fcntl(s->sock_fd, F_GETFL, 0);
if (block)
    delay_flag = orig_delay_flag & (~O_NONBLOCK);
else
    delay_flag = orig_delay_flag | O_NONBLOCK;
if (delay_flag != orig_delay_flag)
    fcntl(s->sock_fd, F_SETFL, delay_flag);

OpenStack Swift using the Eventlet module, which sets the accepted socket non-blocking, resulting in twice the number of fcntl() calls. Not a killer on performance, but it seems simple enough to save a system call here.

Thanks for your consideration,

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