Heavy I/O blocks FreeBSD box for several seconds

Hartmann, O. ohartman at zedat.fu-berlin.de
Thu Jul 7 23:13:43 UTC 2011

On 07/07/11 09:27, Andriy Gapon wrote:
> on 06/07/2011 21:11 Nathan Whitehorn said the following:
>> On 07/06/11 13:00, Steve Kargl wrote:
>>> AFAICT, it is a cpu affinity issue.  If I launch n+1 MPI images
>>> on a system with n cpus/cores, then 2 (and sometimes 3) images
>>> are stuck on a cpu and those 2 (or 3) images ping-pong on that
>>> cpu.  I recall trying to use renice(8) to force some load
>>> balancing, but vaguely remember that it did not help.
>> I've seen exactly this problem with multi-threaded math libraries, as well.
> Exactly the same?  Let's see.
>> Using parallel GotoBLAS on FreeBSD gives terrible performance because the
>> threads keep migrating between CPUs, causing frequent cache misses.
> So Steve reports that if he has Nthr>  Ncpu, then some threads are "over-glued"
> to a particular CPU, which results in sub-optimal scheduling for those threads.
>   I have to guess that Steve would want to see the threads being shuffled between
> CPUs to produce more even CPU load.
> On the other hand, you report that your threads keep being shuffled between CPUs
> (I presume for Nthr == Ncpu case, where Nthr is a count of the number-crunching
> threads).  And I guess that you want them to stay glued to particular CPUs.
> So how is this the same problem?  In fact, it sounds like somewhat opposite.
> The only thing in common is that you both don't like how ULE works.
> ULE has many knobs to tune its behavior.  Unfortunately they are not very well
> documented and there are too many of them.  So, it's not easy to find which
> combination would be the best for a particular work-load.  In your particular
> case you might want to try to increase value of kern.sched.affinity to increase
> affinity of threads to their CPUs.
Not all of those using FreeBSD are developer or experts, even experts of 
a very specific
area of computer science and engineering or  a particular subject of the
FreeBSD kernel and its techniques of scheduling. I'm not capable of 
tuning my servers via
a lot of undocumented knobs, I'm sorry. I'd like to do if there would be 
a kind of howto

> Also, please note that FreeBSD support in GotoBLAS is not equivalent to Linux
> support as I have pointed out before.  On Linux they bind their threads to CPUs
> to avoid the situation that you describe.  Apparently they didn't know how to do
> CPU-binding on FreeBSD, so this is not implemented.  You may have a motivation
> to help them out with this.

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