Edward Ned Harvey wrote: > > I know this seems counterintuitive - but - I have experience to the > contrary. In traditional thinking, of course, swap is slower so you don't > want to use it, but in modern thinking, having swap available boosts your > system performance because the system can trade swap for cache. > > Here's the reasoning: > > At all times, the kernel will grow to the maximum available ram > (buffering/caching disk reads/writes). So obviously the more memory > available, the better, and the less required in user space the better... > but ... This means at all times the kernel is choosing which disk blocks to > keep in cache, as user processes grow, whatever is deemed to be the least > valuable cached disk block is dropped out of ram. > > If you have plenty of swap available, it gives the kernel another degree of > freedom to work with. The kernel now has the option available to page out > some idle process that it deems to be less valuable than the cached disk > blocks.
Right, and that's why I mentioned the swappiness mess on Linux (and I think "maxperm" on AIX, it's been a long time). As far as I am concerned, what you are talking about is fine on a server that is primarily used as a file server, assuming those still exist, but if I run any kind of application, then: I am more than happy for the kernel to use any memory NOT used by an apps to cache the file system I do not want, under any circumstances, the kernel to free the application from memory at the profit of cache. I ended learning about these memory tweak when I witnessed these behaviours: -applications servers being slow every week, or every few weeks, for what seems a few hours in the morning. After investigation, it seemed to correlate with full backup, the morning after the full backup, the apps would be slow. Killing the backup client in the morning, would not help. Think about it, the backup reads the disk, a lot, while the applications are not used, so the kernel frees up both apps and data from memory so it can cache the file system really well (cache using LRU make this problem worse). -Linux workstations with default value for swappiness: you minimize your email client, do a bunch of work, try to bring the email client up, and, it takes 4 or 5 seconds to come up, while you can hear the disk chugging. Worse, you take a phone call for ten minutes, your screen saver/lock kicks in, once you are done the machine seems frozen for a few minutes (all the apps were idle, even without any i/o activity, the memory for the apps got freed up). The only case I can think of swappiness > 0 making any sense is if you start start a lot of applications, but only use a few, and do not change from apps to apps very often. -- Yves. http://www.sollers.ca/ _______________________________________________ Tech mailing list [email protected] http://lopsa.org/cgi-bin/mailman/listinfo/tech This list provided by the League of Professional System Administrators http://lopsa.org/
