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

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

Jens Axboe wrote:
> On Tue, Oct 24 2006, Martin Peschke wrote:
> > Jens Axboe wrote:
> > > > Our tests indicate that the blktrace approach is fine for performance
> > > > analysis as long as the system to be analysed isn't too busy.
> > > > But once the system faces a consirable amount of non-sequential I/O
> > > > workload, the plenty of blktrace-generated data starts to get painful.
> > > Why haven't you done an analysis and posted it here? I surely cannot fix
> > > what nobody tells me is broken or suboptimal.
> > Fair enough. We have tried out the silly way of blktrace-ing, storing
> > data locally. So, it's probably not worthwhile discussing that.
>
> You'd probably never want to do local traces for performance analysis.
> It may be handy for other purposes, though.

"...probably not worthwhile discussing that" in the context of performance
analysis.

> > > I have to say it's news to
> > > me that it's performance intensive, tests I did with Alan Brunelle a
> > > year or so ago showed it to be quite low impact.
> > I found some discussions on linux-btrace (Feburary 2006).
> > There is little information on how the alleged 2 percent impact has
> > been determined. Test cases seem to comprise formatting disks ...hmm.
>
> It may sound strange, but formatting a large drive generates a huge
> flood of block layer events from lots of io queued and merged. So it's
> not a bad benchmark for this type of thing. And it's easy to test :-)

Just wondering to what degree this might resemble I/O workloads run
by customers in their data centers.

> > > You'd be silly to locally store traces, send them out over the network.
> > Will try this next and post complaints, if any, along with numbers.
>
> Thanks! Also note that you do not need to log every event, just register
> a mask of interesting ones to decrease the output logging rate. We could
> so with some better setup for that though, but at least you should be
> able to filter out some unwanted events.

...and consequently try to scale down relay buffers, reducing the risk of
memory constraints caused by blktrace activation.

> > However, a fast network connection plus a second system for blktrace
> > data processing are serious requirements. Think of servers secured
> > by firewalls. Reading some counters in debugfs, sysfs or whatever
> > might be more appropriate for some one who has noticed an unexpected
> > I/O slowdown and needs directions for further investigation.
>
> It's hard to make something that will suit everybody. Maintaining some
> counters in sysfs is of course less expensive when your POV is cpu
> cycles.

Counters are also cheaper with regard to memory consumption. Counters
are probably cause less side effects, but are less flexible than
full-blown traces.


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/