Re: [PATCHES] SIGPIPE handling

[Manfred Spraul]

Bruce Momjian wrote:

Here is my logic --- 99% of apps don't install a SIGPIPE signal handler,
and 90% will not add a SIGPIPE/SIG_IGN call to their applications.  I
guess I am looking for something that would allow the performance
benefit of not doing a pgsignal() call around very send() for the
majority of our apps.  What was the speed improvement?
 

Around 10% for a heavily multithreaded app on an 8-way Xeon server. Far 
less for a single threaded app and far less for uniprocessor systems: 
the kernel must update the pending queue of all threads and that causes 
lots of contention for the (per-process) spinlock that protects the 
signal handlers.


Granted, we need to do something because our current setup isn't even
thread-safe.  Also, how is your patch more thread-safe than the old one?
The detection is thread-safe, but I don't see how the use is.
First function in main():

signal(SIGPIPE, SIG_IGN);
PQsetsighandling(1);
This results in perfectly thread-safe sigpipe handling. If it's a 
multithreaded app that needs correct correct per-thread delivery of 
SIGPIPE signals for console IO, then the libpq user must implement the 
sequence I describe below.

 If you
still pgsignal around the calls, I don't see how two threads couldn't
do:
        thread 1                        thread 2
        --------                        --------
        pgsignal(SIGPIPE, SIG_IGN);
                                        pgsignal(SIGPIPE, SIG_DFL);
                                        send();
                                        pgsignal(SIGPIPE, SIG_DFL);
	send();
	pgsignal(SIGPIPE, SIG_DFL);
	
This runs thread1 with SIGPIPE as SIG_DFL.  
 

Correct. A thread safe sequence might be something like:

pthread_sigmask(SIG_BLOCK,{SIGPIPE});
send();
if (sigpending(SIGPIPE) {
   sigwait({SIGPIPE},);
}
pthread_sigmask(SIG_UNBLOCK,{SIGPIPE});
But this sequence only works for users that link against libpthread. And 
the same sequence with sigprocmask is undefined for multithreaded apps.

--
   Manfred
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