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
>>
>