Re: (ITS#3471) multiple syncrepl clients blocks other syncrepl clients

[Howard Chu <hyc@symas.com>]

hyc@symas.com wrote:

> quanah@stanford.edu wrote:
>  
>
> > --On Friday, January 07, 2005 8:04 PM +0000 openldap-its@OpenLDAP.org wrote:
> >
> > Just to follow up, this scenario actually blocks *all* other incoming 
> > connections (i.e., you can only have two active connections, everything 
> > else times out).
> Since the problem is not occurring on Linux, it appears this is a 
> Solaris-specific issue. It sounds very similar to the problem in 
> ITS#3340. Not clear whether it's thread related or something else.
Well, this problem turns out to be quite Solaris-specific. First of all, 
once the server got busy, it stopped listening to incoming connections. 
Attaching with gdb showed that the main listener thread was waiting in 
thread_yield (ldap_pvt_thread_yield) (see line slapd/daemon.c:1720) 
after having returned from select() with a result of zero (timeout). 
Apparently the listener thread was not getting back onto the CPU at all 
while the other threads were running, so it never got back around to 
select() for new connections again. (The yield at daemon.c: 1969 also 
seems to be a problem.)

Commenting out this yield call seems to have helped that part of the 
problem. We then found that even though the connections stopped timing 
out, there still wasn't very good response from slapd. Attaching with 
gdb at this point showed multiple threads stuck in a mutex_lock invoked 
from the syslog() function.

I think we may need these yield() calls on a single-CPU machine, to make 
sure that incoming operations aren't starved for CPU time. But the test 
machine is a multi-CPU machine, so that kind of consideration doesn't 
apply. Also, since this piece of code just loops back to select(), we 
probably can avoid the yield's altogether when we know that select 
yields. configure already tests for that, so perhaps we can just 
surround these calls with #ifndef HAVE_YIELDING_SELECT.

Comments?

--
 -- Howard Chu
 Chief Architect, Symas Corp.       Director, Highland Sun
 http://www.symas.com               http://highlandsun.com/hyc
 Symas: Premier OpenSource Development and Support