Re: slapd responds very slowly when cpu has 100% usage (but actually low load)

[Howard Chu <hyc@symas.com>]

Antonis Christofides wrote:
> Hi,
>
> At the almost idle Dual Core machine which runs slapd, I run:
>
>    nice sh -c 'while true; do true; done' &
>    nice sh -c 'while true; do true; done' &
>
> (i.e. I'm running this twice).  Then each of the two CPUs always has
> some job to do, so both CPUs have 100% usage, but this is "nice".
>
> Then, slapd takes too long to respond to queries.  It may take 10 or
> 20 seconds.  If I kill or stop one of the two dummy processes, it
> replies instantly.  If I continue both dummy processes, it's back to
> 10 or 20 seconds.  Needless to say all machine resources seem ok; low
> disk usage, lots of spare memory; and slapd is not niced.
>
> If it's not something immediately obvious, could you help me debug it?
> I've run slapd with various "-d" options but it gives me results that
> I have trouble understanding.
>
> The OS is Debian 3.1 (Sarge), with a 2.6.12 SMP Linux kernel.
You didn't mention what version of slapd you're running. On the current 
version (2.3.30) with my dual-core system your test only slowed slapd 
down by 5%.

--
  -- Howard Chu
  Chief Architect, Symas Corp.  http://www.symas.com
  Director, Highland Sun        http://highlandsun.com/hyc
  OpenLDAP Core Team            http://www.openldap.org/project/