Re: sysmap_64bit: rmap ovflo, lost

Don Morris wrote:
> joel-garry_at_home.com wrote:

Sorry I didn't notice this post earlier, google sometimes doesn't show everything without some prodding. Thanks, it was very helpful! A couple of interspersed comments for the future curious:

> > helpful gurus:
> >
> > hp-ux 11.11 4 processor rpr3340 box crashed last night. Trying to
> > figure out how to prevent this in the future. Oracle 9.2.0.6.
> >
> > The uptime was a little over 2 months. Looking at syslog, I see lots
> > (>17K lines) of:
> > Apr 25 20:47:34 ZEUS vmunix: sysmap_64bit: rmap ovflo, lost
> > [68419543,68419559)
>
> Ouch.. kernel virtual address space is so fragmented it is getting
> dropped on the floor.
>
> Next chance you get, double the nsysmap64 kernel tunable -- that's
> a workaround... not a definite fix.

Workaround better than blind reboots!

>
> More information:
> http://docs.hp.com/en/TKP-90202/re68.html

"pathological workloads" I love it! Now why didn't I find this when I searched for "rmap ovfl?" Yet another mystery of the universe. Rule of Thumb in docs... (that's a smiley for some of us Oracle folk, there have been heated discussions about it regarding configuration and performance tuning).

>
> >
> > They started at the exact time my Oracle RMAN backup started. The
> > script that does the backup does a number of things, such as remove old
> > backup files, run the RMAN script (nocatalog), then compress some of
> > the backup files onto an nfs device. The RMAN completed, system
> > crashed during compress.
>
> My bet? This backup results in lots of little I/O buffers being
> created which are freed in a very asynchronous manner -- with more than
> a few taking a *long* time to be freed.

That sounds like a good bet. We also have transaction processors which run continuously, I don't know what they are doing, but I'm always suspicious of such things. Support calls to both vendors involved (app and tp) asking about memory leaks or fragmentation have been met with resounding silence.

>
> The easiest way for the kernel virtual address space to get fragmented
> is to have lots of little dynamic allocations be made... and then lots
> of little pieces to be freed back which can't form their original larger
> ranges (coalesce) because pieces are missing. I/O buffers (being small
> bits of kernel dynamic memory) are unsurprisingly good at this.

What? No garbage collection? Ah well, not for me to question the gods.

>
> >
> > Looking at
> > http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=70397
> > (neither of the links in there work for me), I now have the idea that
> > something fragmented kernel memory. But what? I was about to write a
> > script to periodically capture the largest processes while RMAN is
> > running, but then I started wondering if it is not really RMAN, but
> > something previous to RMAN that sets up the problem. Looking again at
> > syslog, I see the rmaps happening on a few days in April at various
> > times during the day, once during production day and 7 times off-hours
> > (sometimes during RMAN, sometimes during compress), April 10-15, but no
> > other times since boot. If it were RMAN, wouldn't I see the problem
> > whenever RMAN ran? And why this time did it go nuts and crash the
> > system, but not the other times?
>
> You only see the problem when the data structures to hold free kernel
> virtual address space overflow. In other words, you may be fragmented
> every time RMAN runs -- but just not fragmented *enough*. Alternately,
> it may be that you have an intermittent I/O timeout or somesuch which
> causes some runs of RMAN to hold on to every Nth buffer for a long time
> and cause the critical fragmentation problem -- where your other runs
> manage to release the buffers together and they coalesce back up without
> much fragmentation.

Yes, we've also seen intermittent I/O timeouts during times of heavy I/O (not just rman, but also compressing files and high database redo activity), all advice about that has been not to worry about it. I've been trying to use this as an argument to move away from RAID-5, but that's another discussion. What you are saying here ties lots of little bits of evidence together.

>
> >
> > Using the 'UNIX95= ps -e -o "vsz args" |sort' command, I see that some
> > third party application processes get big: 132640K is the biggest just
> > now, (those are killed off nightly if the users forget to log off - but
> > later than this backup). So I tried 'ps -efl|sort -nk10|tail -10' ,
> > which shows that same process as 29527 pages (and lets me see exactly
> > who it is). But I don't quite get what vsz and sz are telling me, I
> > guess I need to subtract some shared memory? man ps isn't too clear.
>
> User virtual address space is managed completely differently. ps isn't
> going to help you here at all... (although if you're running lots and
> lots of processes, that can consume a lot of dynamic memory to manage
> the process metadata -- and can lead to fragmentation as well. Hence why
> nsysmap64 defaults to being based off of nproc for nproc > 800).

Thanks for clarifying that. I've noticed Oracle's recommendations for nproc vary over time.

>
> >
> > I don't see how to figure which process is fragmenting memory. Don't
> > have glance. Should I be looking for processes that get bigger and
> > smaller, rather than the largest? There is a transaction monitor that
> > appears to be doing that. Or should I watch for something continually
> > growing? I don't know of anything that has changed on this system
> > specific to this month, and don't really see how a memory leak could
> > come and go and come back big when users and cron do the same thing
> > day-to-day.
>
> You really don't have the tools to find this -- HP Support does. You
> should have gotten a dump when the panic occurred, I highly recommend
> using your support channels to track down the root cause of the
> fragmentation and see what they recommend.

Noted. I think that stuff all got turned off by habit from the days of K-class and Autoraid.

>
> >
> > Is it really going to be necessary to reboot this thing monthly?
>
> You can mitigate it by increasing nsysmap64 as mentioned above -- but
> you still need Support to help figure out the root cause to make sure
> increasing the sysmap capacity isn't just applying a bandaid.

It's posts like yours that really make me appreciate usenet. I can't thank you enough.

>
> Don
>

jg

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