Re: onmode -F

In article <33A6805F.167E@uk.ibm.com>, Daniel Williams <danny@uk.ibm.com> writes >Zeljan Silje wrote: >> >> Hi, >> >> in "Performance Guide Version 7.2" I found recommandation to run onmode >> -F using cron facility. >> Does anyone has experience with this, and how often should I execute >> onmode -F using cron?>> I am using Informix OnLine7.22 on AIX4.2. >> >> Thanks in advance, >> Zeljan >> > >onmode -F frees unused memory segments> >The answer depends entirely on how your system is being used (like all >performance >and tuning questions). > >Look to see how often your system is allocating additional memory segments (in >the >online log). The message will be something like: > >08:32:42 dynamically allocated new shared memory segment (size 8388608)> >Run onstat - to find out the resident memory size. > >Next, run an onmode -F followed by an onstat - to find out the new resident >size. >If it is significantly lower, then consider running onmode -F every night (or >when >the system load is low if night is a bad time). > >Also look at the output of onstat -g seg (see manual). > >If you find that there is a lot of additional memory being allocated but not >being >released with an onmode -F then it is likely that it is being utilised. If so, >consider increasing the SHMADD $ONCONFIG parameter, to allow informix to >allocate >more memory in one operation. Conversely, if memory is being allocated but is >available for release soon after, it is possible that the total allocation is >not >being used. Then consider decreasing the SHMADD parameter. > >You should also examine the system from the AIX perspective, using the ipcs >command, to check that any shared memory used by informix for interprocess >communcation is being released correctly. > >As you can see, the answer is not a simple one and requires knowledge of both >how >the both OnLine engine and AIX operate. > >Hope this information is useful. > >Dan > > >P.S. (To all readers) To the best of my knowledge this reply is correct. If it >is >not, please post corrections. All good, but... I would add that if onmode -F tends to free segments then some jobs/queries use a lot of memory and then are able to stop using it. This means there is a peak in memory usage.. check that one query is not doing a dynamic hash join (sequential scan) which requires a lot of memory for the join rather than using an index. i.e. AN INDEX IS MISSING OR A QUERY NEEDS REWRITING.. -- David Williams