I can tell you my personal experience with SWAP/OOM and RHEL under both 32 and 64 bit and you can judge as to whether or not its any good in your situation; During a migration from 32 bit SLES to 32 bit RHEL I discovered that workloads previously relatively functional under SLES started to experience issues with OOM kills; In the beginning I was a bit confused about the whole idea; The new machine had 3 times the amount of memory the old machines had; 10 times more powerful processors and 7 years of technology enhancements; How is this possible? Well it turns out that badly written code is badly written code no matter what the conditions but that above and beyond the point. On our old machines we were running a SMP based kernel and 16GB of RAM; The workload was mainly batch processing style, a lot of things waking up doing their thing and back to sleep the go, all day and night long. The new machines were now running RHEL 5.4 with a PAE Kernel. First test begin all looks good; Then we started adding some real data crunching and load and the OOM's started; I had roughly 48GB of RAM and a 8GB swap; figured that should be good; No sir. After a lot of research and talking it RedHat we were advised that under RHEL 5 support of Hugemem kernels was dropped; Guess that is their way of saying *silently* goodbye 32 bit; We were advised that the support configuration under RHEL 5 32 bit is up to 16GB of RAM; Thats it if we wanted to guarantee stable operation; BTW the reason we were OOM killed was due to Low Mem zone pressure; Finally with 16GB of RAM, 12GB of swap and quite some tuning (reservations for Low zone etc) we got the machine at a almost stable state (we still get the occasional runaway process which will cause a OOM of some other Random process); Not happy to say the least; Oh and BTW one of the other tunables I had to mess with is the swappiness value; set it to 100 seems to work ok for us.
Now I run Database servers on RHEL 5 64 bit; Thats a different beast; with no High/Low zone memory separation (I believe under 64 bit all memory is in the low zone) and a application that can leverage hugepages things are much much better; on the 32 bit side I am still bothered by this whole pain of OOM; I am not sure what else I can do from a SA perspective. so in a way I am also extremely interested in this thread and any feedback anyone can provide. best regards On Wed, Apr 20, 2011 at 8:13 AM, Dan Foster <[email protected]> wrote: > Hot Diggety! Michael C Tiernan was rumored to have written: >> ----- Original Message ----- >> > From: "Adam Tauno Williams" <[email protected]> >> >> > Most recommendations I see [...] >> >> The thing I keep seeing is stuff that discusses about how much the >> system "*needs*". There's lots of sides about how you can live without >> much or you *should*[1] have some or you *should*[1] have lots but I'm >> wondering if anyone found any discussions about what happens if you >> have *too* much swap? >> >> Ignoring the issue of wasting disk space, are there any negatives to >> having lots of swap? > > That's actually easy to answer: the system slows down BIG time if it's > heavily utilized, to the point where you're forced to power cycle it > uncleanly if you want to regain control in less than 24 hours. :) > > So swap sizing is bit of an art; need to know what the app needs (and > how it works behind the scenes) and allocate accordingly, but not set up > such so huge of an 'overdraft' account that you find yourself in an > impossible-to-sanely-fulfill-quickly situation. > > -Dan > _______________________________________________ > Tech mailing list > [email protected] > https://lists.lopsa.org/cgi-bin/mailman/listinfo/tech > This list provided by the League of Professional System Administrators > http://lopsa.org/ > -- Paul _______________________________________________ Tech mailing list [email protected] https://lists.lopsa.org/cgi-bin/mailman/listinfo/tech This list provided by the League of Professional System Administrators http://lopsa.org/
