From - Mon Aug 04 07:36:56 1997
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From: Dave Butenhof <butenhof@zko.dec.com>
Newsgroups: comp.os.vms
Subject: Re: UCX & PThreads Performance Problems
Date: Fri, 01 Aug 1997 10:04:50 -0400
Organization: Digital Equipment Corporation
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Phil Penney (YRL) wrote:
> 
> We're using DECthreads POSIX 1003.1c in an application on an Alpha
> running VMS V7.1 and UCX V4.1 ECO Level 4.
> 
> I notice an unacceptable delay when using condition variables which are
> taking 0.45 seconds elapsed time from a pthread_cond_signal() in one
> thread to the waking of a second thread sitting on pthread_cond_wait().
>
> 2 other threads are waiting on sockets using the recvfrom() function with
> no flags set - nothing arrives at the sockets while the condition variable
> is being tested. When I remove these 'socket' threads from the application
> the condition variable delay is 0.06 seconds.

Assuming that upcalls are enabled, "waiting on a socket" should end up
making the thread wait on a condition variable within the thread
library. However, if upcalls are not enabled (either for the system or
for the image), then a blocking kernel call will stall the PROCESS (and
all thread) until a timeslice AST occurs. My guess is that this is
what's happening.

> Has anyone used the combination of pthread condition variables and socket
> functions without noticing performance problems? Obviously any suggestions
> would be gratefully received!

Check the MULTITHREAD sysgen parameter. If it's set to 0, both upcalls
and kernel threads are disabled for the system. If set to 1, upcalls are
enabled, but multiple kernel threads are disallowed. Otherwise, the
value is the number of kernel threads created per process (which should
usually be the number of processors in the system).

In addition, the main image must be linked with
	/THREADS_ENABLE=(UPCALLS,MULTIPLE_KERNEL_THREADS)
If it's not, then a process running that image will be unable to create
multiple kernel threads, or avoid blocking the process on kernel calls,
regardless of the MULTITHREAD parameter. If you need to change those
flags in an image that you've already built, try this:

$ set command sys$update:threadcp
$ threadcp <image>/enable=(upcalls,multiple_kernel_threads)

	/dave

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