On Jan 30, 2006, at 03:50, Howard Chu wrote:
Helge Hafting wrote:
linux-os (Dick Johnson) wrote:
To fix the current problem, you can substitute usleep(0); It will
give the CPU to somebody if it's computable, then give it back to
you. It seems to work in every case that sched_yield() has mucked
up (perhaps 20 to 30 here).
Isn't that dangerous? Someday, someone working on linux (or some
other unixish os) might come up with an usleep implementation
where usleep(0) just returns and becomes a no-op. Which probably
is ok with the usleep spec - it did sleep for zero time . . .
We actually experimented with usleep(0) and select(...) with a
zeroed timeval. Both of these approaches performed worse than just
using sched_yield(), depending on the system and some other
conditions. Dual-core AMD64 vs single-CPU had quite different
behaviors. Also, if the slapd main event loop was using epoll()
instead of select(), the select's used for yields slowed down by a
couple orders of magnitude. (A test that normally took ~30 seconds
took as long as 45 minutes in one case, it was quite erratic.)
It turned out that most of those yield's were leftovers inherited
from when we only supported non-preemptive threads, and simply
deleting them was the best approach.
I would argue that in a non realtime environment sched_yield() is not
useful at all. When you want to wait for another process, you wait
explicitly for that process using one of the various POSIX-defined
methods, such as mutexes, condition variables, etc. There are very
clearly and thoroughly defined ways to wait for other processes to
complete work, why rely on usleep(0) giving CPU to some other task
when you can explicitly tell the scheduler "I am waiting for task foo
to release this mutex" or "I can't run until somebody signals this
condition variable".
Cheers,
Kyle Moffett
--
Unix was not designed to stop people from doing stupid things,
because that would also stop them from doing clever things.
-- Doug Gwyn
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