linux-kernel - Re: Linux Kernel Markers - performance characterization with large IO load on large-ish system

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Message-ID: <20071009173127.GA14714@Krystal>
Date:	Tue, 9 Oct 2007 13:31:27 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Alan D. Brunelle" <Alan.Brunelle@...com>,
	linux-kernel@...r.kernel.org,
	btrace <linux-btrace@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Linux Kernel Markers - performance characterization with large IO load on large-ish system

* Ingo Molnar (mingo@...e.hu) wrote:
> 
> * Alan D. Brunelle <Alan.Brunelle@...com> wrote:
> 
> >  o  All kernels start off with Linux 2.6.23-rc6 + 2.6.23-rc6-mm1
> > 
> >  o  '- bt cfg' or '+ bt cfg' means a kernel without or with blktrace 
> > configured respectively.
> > 
> >  o  '- markers' or '+ markers' means a kernel without or with the 
> > 11-patch marker series respectively.
> > 
> > 38 runs without blk traces being captured (dropped hi/lo value from 40 runs)
> > 
> > Kernel Options       Min val    Avg val    Max val    Std Dev
> > ------------------  ---------  ---------  ---------  ---------
> > - markers - bt cfg  15.349127  16.169459  16.372980   0.184417
> > + markers - bt cfg  15.280382  16.202398  16.409257   0.191861
> > 
> > - markers + bt cfg  14.464366  14.754347  16.052306   0.463665
> > + markers + bt cfg  14.421765  14.644406  15.690871   0.233885
> 
> actually, the pure marker overhead seems to be a regression:
> 
> > - markers - bt cfg  15.349127  16.169459  16.372980   0.184417
> > + markers - bt cfg  15.280382  16.202398  16.409257   0.191861
> 
> why isnt the marker near zero-cost as it should be? (as long as they are 
> enabled but are not in actual use) 2% increase is _ALOT_. That's the 
> whole point of good probes: they do not slow down the normal kernel.
> 
> _Worst case_ it should be at most a few instructions overhead but that 
> does not explain the ~2% wall-clock time regression you measured here.
> 
> So there's something wrong going on - either markers have unacceptably 
> high cost, or the measurement is not valid.
> 
> 	Ingo

Hi Ingo,

Tests were executed in the following conditions:

"Taking Linux 2.6.23-rc6 + 2.6.23-rc6-mm1 as a basis, I took some sample 
runs of the following on both it and after applying Mathieu Desnoyers 
11-patch sequence (19 September 2007).

   * 32-way IA64 + 132GiB + 10 FC adapters + 10 HP MSA 1000s (one 72GiB
     volume per MSA used)"

Even though the 19 Sept. 2007 markers were released with dependency on
immediate values, there are no optimized immediate values currently
available on ia64. Therefore, we add a d-cache hit for every marker
until we merge immediate values and implement the ia64 optimization.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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