[Info-vax] Raxco VMS Tuning Seminar Notes

[AEF]

On Nov 8, 3:23 pm, jls <notva... at yahoo.com> wrote:
> On Fri, 5 Nov 2010 04:31:20 -0700 (PDT), Neil Rieck
>
>
>
> <n.ri... at sympatico.ca> wrote:
> >On Nov 3, 1:43 pm, jls <notva... at yahoo.com> wrote:
>
> >[...snip...].
>
> >AFAIAC, the problem with the confusion around VMS tuning is due to the
> >fact that many concepts are spread across multiple VMS manuals. So
> >yesterday I was reading semi-unofficial VMS docs like "OpenVMS System
> >Management Guide" which also gives the impression that AWSA runs
> >independently every so often (like WATCHDOG or HITMAN). See topic
> >18.7.3.2 or click the following link:
>
> >http://books.google.ca/books?id=yn9RN2e32J4C&pg=PA421&lpg=PA421&dq=aw...
>
> >But this official document "VMS System Performance Management" (see
> >topic: 3.5.6 titled "Voluntary Decrementing")
> >http://h71000.www7.hp.com/doc/73final/6491/6491pro_003.html
> >discusses PFRATL in a context which has nothing to do with swapper
> >trimming but everything to do with the process limiting itself.
> >itself. So thanks for setting me straight.
>
> Glad to help.
>
>
>
> >###
>
> >Okay so what would a system look like with with AWSA-
> >Contraction(Reduction) disabled?
> >Well, if the system becomes short of memory then swapper trimming
> >would knock some/all processes who have borrowed memory over quota
> >back to quota. If the system is still short of memory then some procs
> >would be reduced even further to SWPOUTPGCNT (unless it was manually
> >set higher) prior to swapping out.
>
> Again, as I explained, judicious use of swpoutpgcnt so that it
> encourages swapping entire processes, including up to their wsextent,
> can actually help performance by reducing the amount of overhead cpu
> spent on memory management, and also stop process page thrashing.
>
> The assumption is that you're not trying to get  10 lbs of active sh*t
> in a 5 lb bag.  I.e., you have a max 5 lbs of *active* stuff in that
> bag, but can take on 10 (or 100) lbs of other inactive stuff.  Just as
> long as you don't get more than 5 lbs of active stuff (since you've
> only got enough physical resources to handle the 5).
>
>
>
> >So I think it is better to have processes reducing their own memory
> >requirements (acting like good citizens in the system) provided it
> >doesn't invoke too much thrashing (hard or soft).
>
> If it were idle processes giving up pages, then I'd agree with you.
> But this isn't what is happening with AWSA.  With awsa, processes
> aren't reducing their memory when they don't need it.  They are being
> forced to give up necessary pages in their workingset by virtue of
> actively using them.
>
>
>
> >How could you reduce AWSA thrashing?
> >1) Increase AWSMIN so you don't surrender below a certain value
> >2) Reduce the size of WSDEC to a value below WSINC (the default SYSGEN
> >values are the other way around). This would result in process being
> >able to get more memory when it was required but surrendering fewer
> >pages when required. Don't worry, if you get this wrong then then you
> >can always fall back on swapper trimming
> >3) Make AWSTIME some small multiple of QUANTUM. (don't make it too
> >high as this would also affect AWSA-Expansion)
>
> The point is that AWSA is a very drastic way to manage memory in most
> cases.  It trades off a lot of CPU to perform memory management
> activities compared to other methods.
>
>
>
> >But all of this is moot if you've got plenty of memory like I do on my
> >AS-DS20e with 3 GB. It is a different story on my AS-2100 which only
> >has 128 MB
>
> Right, the only reason to do more agressive memory management /
> reclamation is when you're in a memory starved environment.  However,
> just out od curiosity, do you use AWSA on the DS20e?  According to the
> RAXCO bible, it was encouraged for use in ALL (or nearly all)
> circumstances.
>
> Look at all those processes on the AS-2100 and check how idle most of
> them are.  How many are COM at any one time (well, say in a 1 min
> interval).  How much memory would it take to keep all those COM
> processes happy simultaneously.
>
> Calculate the probability that a process that is idle for, say, one
> minute is going to become active in the next minute.  If you don't
> like one minute, choose 5 min.  Or whatever floats your boat.  But
> calculate the probability of them become compute bound again after
> that period.
>
> In many workloads, those probabilities are pretty low.  That means
> that if the entire process was swapped out (swpoutpgcnt = wsextent),
> then you have much more memory available than stealing a couple of
> pages here-and-there.
>
> And the active processes can get more work done because they're not
> thrashing.  If and when the idle process does become active again, it
> is swapped back in with one I/O.
>
> Swapper trimming can be a significantly lower overhead for many
> circumstances.  

Can you comment on the

3.7 Active Memory Reclamation from Idle Processes

and how it compares to your methods?

AEF