Re: performance loss on real time linux

[Pierangelo Masarati <ando@sys-net.it>]

Dieter Kluenter wrote:
> Hi,
> I am at an early stage of deploying OpenLDAP running on Monstavista
> Carrier Grade linux, with Linux-2.6 kernel but modified scheduler.
> http://www.mvista.com
> For this I compiled and installed OpenLDAP-2.3.27 on my TP T43
> notebook and on a x86 Carrier Grade hardware, I'm not sure about the
> processor manufacturer and type of this hardware.
> On both machines identical DB_CONFIG files, slapd.conf and data have
> been set up. I created only 10,000 entries as this would the expected
> average database size be.
> A ldapsearch on localhost with SuSE-9.3 Linux, after some warming
> up the results were levelling at
>
> real    0m0.010s
> user    0m0.002s
> sys     0m0.003s
>
> with Montavista 
>
> real    0m0.827s
> user    0m0.014s
> sys     0m0.015s
>
> Due to a real time scheduler I was expecting the results to be vice
> versa.
>   
> Can a RT scheduler or other modifications to the operating system have
> a negative influence on performance of OpenLDAP?
>   

You can't expect results to be any better than under non-RT os, since a 
RT-scheduler will always add overhead to non-RT processes since it will 
preempt much more than a normal OS.  I have no experience with 
Monta-Vista, however my institution (Politecnico di Milano) develops 
RTAI, the Real-Time Application Interface for Linux 
(http://www.rtai.org/); I'll check what happens when running OpenLDAP 
with that.

In the meanwhile, can you tell something more about Monta Vista's 
handling of RT?  Does it include the PREEMPT_RT patch, which will likely 
become part of Linux in a year or so, or does it rather implement some 
custom support to handle preemption?

About your test: are your results an average over a realistic number of 
trials?  Is the database entirely in cache or does it require any access 
to disk?  Mallocs under a real-time scheduler might also represent a 
significant bottleneck; in this case, slapd should really handle memory 
internally, to minimize the amount of system calls.

p.



Ing. Pierangelo Masarati
OpenLDAP Core Team

SysNet s.n.c.
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