[haiku-development] Re: On timeslices and cycles
- From: Urias McCullough <umccullough@xxxxxxxxx>
- To: haiku-development@xxxxxxxxxxxxx
- Date: Fri, 13 Mar 2009 09:03:44 -0700
On Fri, Mar 13, 2009 at 5:41 AM, Ingo Weinhold <ingo_weinhold@xxxxxx> wrote:
> On 2009-03-13 at 13:08:37 [+0100], Christian Packmann
>> And hard affinity is a requirement in some cases. E.g. when I want to get
>> reliable Time Stamp Counters (or PMC) measurements on a Barcelona/Phenom,
>> I need to use hard CPU affinity for a single core, as the CPUs will have
>> varying frequencies and thus TSCs per core. I'm currently considering just
>> switching off all CPUs/cores but one on Haiku, as this is the only way to
>> get reliable measurements.
>
> From what I've read so far, pretty much the only good use for hard affinity
> is for performance measurements. When pinning a thread to one CPU you
> prevent it from running when this CPU is busy, even if another CPU is idle
> at the same time. While CPU hopping is certainly not good, not running is
> usually even worse. Soft affinity seems to be the best strategy in finding
> a good compromise, though it's not trivial to implement.
>
> IMHO, leaving scheduling decisions to application developers is certainly a
> bad choice (cooperative multitasking, anyone?). Allowing them to give the
> OS hints, how their threads will behave wrt. to CPU, memory, I/O usage,
> might be useful, though.
Well, at least most distributed computing apps make use of CPU
affinity (both hard and soft) - since the primary purpose of such apps
is to crunch as hard as possible, it's generally desirable to fix each
thread to a single core. These apps are generally designed to run at
lowest or idle priority so any thread in the system can run over the
top of them. It is not at all desirable to let the thread migrate, as
there is actually no intention to run the thread when the specific cpu
is busy, nor do all threads being run require equal time. Thus some
threads will finish a bit earlier than others, and that's generally
acceptable - and not just on that particular computer since worker
threads for the same project are running on hundreds of thousands of
computers worldwide. :)
I bring this up because in the hardcore distributed computing camp,
the developers and users alike, are always seeking out new ways to
make their client apps run faster on each target OS, and OSes with
"weak" threading control are usually avoided. Most of these
applications have even user-configurable control over the affinity and
thread count.
Before you suggest that Haiku isn't targeting distributed computing, I
will point out that the vast majority of of the work done for
public-run distributed computing projects are done with Windows XP at
this point.
- Urias
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