Re: [Patch 0/5] I/O statistics through request queues

[Martin Peschke <mp3@de.ibm.com>]

Well, the instrumentation "on demand" aspect is half of the truth.
A probe inserted through kprobes impacts performance more than static
instrumentation.


Phillip Susi wrote:
> Would this be a good candidate to implement using kprobes?  I was under 
> the impression that basing instrumentation on kprobes would be a good 
> thing since you can load the instrumentation code only when needed, then 
> unload it.
>
> Martin Peschke wrote:
> > This patch set makes the block layer maintain statistics for request
> > queues. Resulting data closely resembles the actual I/O traffic to a
> > device, as the block layer takes hints from block device drivers when a
> > request is being issued as well as when it is about to complete.
> >
> > It is crucial (for us) to be able to look at such kernel level data in
> > case of customer situations. It allows us to determine what kind of
> > requests might be involved in performance situations. This information
> > helps to understand whether one faces a device issue or a Linux issue.
> > Not being able to tap into performance data is regarded as a big minus
> > by some enterprise customers, who are reluctant to use Linux SCSI
> > support or Linux.
> >
> > Statistics data includes:
> > - request sizes (read + write),
> > - residual bytes of partially completed requests (read + write),
> > - request latencies (read + write),
> > - request retries (read + write),
> > - request concurrency,
> >
> > For sample data please have a look at the SCSI stack patch or the DASD
> > driver patch respectively. This data is only gathered if statistics have
> > been enabled by users at run time (default is off).
> >
> > The advantage of instrumenting request queues is that we can cover a
> > broad range of devices, including SCSI tape devices.
> > Having the block layer maintain such statistics on behalf of drivers
> > provides for comparability through a common set of statistics.
> >
> > A previous approach was to put all the code into the SCSI stack:
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=115928678420835&w=2
> > I gathered from feedback that moving that stuff to the block layer
> > might be preferable.
> >
> > These patches use the statistics component described in
> > Documentation/statistics.txt.
> >
> > Patches are against 2.6.19-rc2-mm2.
> >
> > [Patch 1/5] I/O statistics through request queues: timeval_to_us()
> > [Patch 2/5] I/O statistics through request queues: queue instrumentation
> > [Patch 3/5] I/O statistics through request queues: small SCSI cleanup
> > [Patch 4/5] I/O statistics through request queues: SCSI
> > [Patch 5/5] I/O statistics through request queues: DASD
> >
> > Martin



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