About buffer cache allocation, really dynamic? dbc_max_pct dbc_min_pct

[Hein van den Heuvel · Accepted solution]

>> "They are afraid that 100% memory allocation means no more physical mem for the user procs, but my knowledge was that buffercache is deallocated for sys/user procs mem."

Yes indeed. And that's very effective in general, but can cause performance 'glitches' when the timing is bad, notably on older versions. SEP expresses to be very concerned about this. I'm less concerned, but he clocked more hours with HPUX production systems than I did :-).

The solution is to just try, and force the issue. Dynamically change dbc_max_pct down by a few (2 - 5 procent). That will mimic an increase in user VM demand, and will (best I understand this) reflect the worst case reaction as it gives the cache an immediate need to relinquish versus a gradual increse in pressure in the nor growing process case.

You could also 'play' with a program to malloc (and touch!) 200 - 500 MB of memory and see how the system reacts when at 99% versus when at 90% total user memory.

The delay in my reply was for the want of making the numbers you posted 'add up' and to find a good reference to the meaning of 'Active VM' and the likes. Google and/or The documentation let me down. I could find reference, but did not find any explanation / enumeration of all the columns. Anyone else?

The numbers you posted looked fine, reflecting a happy system (high user to system cpu time ratio, plenty of free pages), but I read your follow up reply as to explain that those numbers were not in the full load situation.

hth,
Hein van den Heuvel
HvdH Performance Consulting

[Tiziano Contorno _]

Thank you!

I hope I will be able to do some test as you suggested (decrease max cache or allocating user memory while at 100%).

I keep it open if anyone wants still contribute and for the test results update.

Regards,
Tiziano.

[Steven E. Protter]

I have very little time with HP-UX 11.31 production. Many hours with 11.11, a reasonable amount with 11.23.

It is true, HP has worked on reducing the performance penalty for a system changing its buffer cache rate.

I noticed the system could make changes on 11.31 without a horrible slowdown, but it was still noticeable when it happened.

It was worse with 11.23 and awful with 11.11.

Having all the system memory allocated is not a problem. It is merely utilizing an asset. The trick is allocating it in a way that is best for your environment.

To assist further, I'd need to know the OS level and what third party applications you intend to run.

SEP