On Wed, Oct 7, 2009 at 11:18 PM, Marcy Cortes <[email protected]> wrote:
[snip] > +--------------------------------------------------------------+ > |echo 0 >/proc/sys/vm/swappiness > +-------------------------------------------------------------+ > This setting may improve or degrade the performance of an application. Since > there is > adequate memory already dedicated to this workload, large amounts of memory > would not > need to be swapped to disk anyway. Since precautionary 'early' swapping is > now avoided, our > results should be free of the effects of this kind of swapping. As I interpret the paper, the author concludes that for this particular workload the setting is appropriate because they have plenty of memory anyway (I am tempted to think that if you have plenty of memory, Linux memory management would not need a helping hand...) > Wouldn't this always be a good thing under VM to force it to reduce its > memory footprint? Are there cases where it wouldn't be? I spent some more time on this lately and I believe that for Linux on z/VM the proper setting would be 0. But I did not have time yet to set up tests to show the benefits with real numbers. And I don't really like presenting theory without the numbers. My motivation for disabling swappiness is not the same as in that paper, but because of the double layer of memory management. It will probably require a slightly larger virtual machine, but the good part of it is that VM can page those parts out. The memory cost of the virtual machine remains the same, but it takes less paging to get there. For this to work well, idle virtual machines must drop from queue. When Linux does not go idle according to VM, memory management is pretty random and you probably will not notice the effect. So I would mainly suggest this for Linux guests that have low utilization and drop from queue when idle. Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/ ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
