Re: [Valgrind-users] cachegrind and multiple cores

[Howard C. <hy...@sy...>]

Josef Weidendorfer wrote:
> On Tuesday 26 September 2006 11:50, Howard Chu wrote:
>> Has anyone written a cache simulator for cachegrind that tracks caches 
>> in separate CPU cores?
> 
> No.
> It would be nice to have it, and it should not be very difficult to
> add to cachegrind/callgrind.
> 
> However, there are two caveats:
> - You have to map VG threads to processors. This needs the simulation
> of some scheduling strategy inside of valgrind. An easy one is
> roundrobin assignment: proc = thread % procnumber, but this usually does
> not match reality because a sane scheduler takes the load of a thread
> into account with the goal of equally distributing threads loads to
> processors.
> - In VG, threads are scheduled in a sequentially order, which does not
> match any reality on a SMP machine where threads can run simultaneously.
> This can have large influences on the number of coherency misses you are
> interested in (false cache sharing). Moreover, VGs scheduling
> interval should be small to get reasonable results, which slowdowns the
> simulation even more.

This seems to be the greater problem, since VG's scheduler only runs one 
thread at a time. Right, using a smaller scheduling interval would 
probably help. We'd want to make sure that each thread executes for 
exactly the same number of cycles, to have any hope of simulating 
simultaneous execution.

>> The main thing I'm interested in would be to see  
>> how often cache line sharing occurs in a multithreaded program.

-- 
   -- 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/