On Sat, 5 Sep 2009, Yves Dorfsman wrote: > 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).
note that there is a flag that the backup software should be using to tell the system that it's not going to be accessing this data again. > -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). what else is running on the system that is asking for memory? the kernel won't throw away memory unless something else is asking for it. > 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. I don't think 0 is the right value, but for a long time the kernel did default to a much to high value, within the last year or so the efault was greatly reduced. David Lang _______________________________________________ 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/
