Re: [Xen-devel] [Patch 2 of 2]: PV-domain SMP performance Linux-part

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxxxxxxx>]

George Dunlap wrote:
> On Mon, Jan 12, 2009 at 12:55 PM, Juergen Gross
> <juergen.gross@xxxxxxxxxxxxxxxxxxx> wrote:
>> Conclusion:
>> -----------
>> Differences not really big, but my "no deschedule" patch had least elapsed
>> time for build-jobs, while scp was able to transfer same amount of data as
>> in slower original system.
>> The "Yield in spinlock" patch had slightly better dbench performance, but
>> interactive shell commands were a pain sometimes! I suspect some problem in
>> George's patches during low system load to be the main reason for this
>> behaviour. Without George's patches the "Yield in spinlock" was very similar
>> to the original system.
> 
> Hmm, the shell performance is a little worrying.  There may be
> something strange going on...
> 
> Without my patches (at least, without the "yield reduces priority"
> patch), "yield" is basically a no-op, so "yield in spinlock" is
> functionally equivalent to the original system.
> 
> According to your numbers, the "user time" and "system time" were
> exactly the same (only 0.6 seconds longer on system time), even though
> the overall build took 52 seconds longer.  Is it possible that the
> "yield" patches actually made it run less often?

I assume this could be possible. Do you think the following is reasonable?

If a vcpu is waiting for a lock it will yield, which will reduce it's
priority. This will increase the latency, if other vcpus are ready to run.
Normally the vcpu waiting for a lock is in some kind of critical path which
might be delayed significantly by the yield.
In sum the complete machine isn't burning cycles spinning, but the code
paths with lock conflicts are the loosers...

> 
> scp works over tcp, which is often sensitive to latency; so it's
> possible that the lowered priority on yield caused "hiccoughs", both
> in the scp connections, and the interactive shell performance.

This sounds reasonable to me.

Juergen

-- 
Juergen Gross                             Principal Developer
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