Re: statistics with intermediate results

[Martin Peschke <mp3 at de dot ibm dot com>]

William Cohen wrote:
> Associative arrays can be used for this purpose. Use the pointer to the 
> data structure as a key for the associative array. Store start time in 
> the associative array. Then when the data structure is encountered for 
> the completion operation fetch the time from the associative array and 
> compute the elapsed time.
>
> How many outstanding operations are there going to be at any given time?

It depends...

For SCSI, there are certain limits for tagged command queueing.
Devices may impose limits, adapters may impose limits, other layers
and subsystems in Linux might impose limits (blocklayer?
SCSI mid layer?).

For the IBM zSeries FCP adapter driver there used to be
(rather arbitrary) limits of 32 concurrent commands per LUN
and 4096 concurrent commands per adapter. Experience shows that
when running some I/O stress workloadt or benchmark, we manage
to hit these limits easily.

In short, I would expect to see up to hundreds or maybe even thousands
of outstanding operations going to be at any given time for systems
like database servers, that is, for systems that are likely candidates
for a performance analysis.

I am a little concerned that searching huge systemtap arrays for
each request could be too expensive. But I don't know much about
the bowels of the systemtap runtime.

Martin