On Wed, 2006-09-13 at 06:23 -0500, Aleksandar Milivojevic wrote: > The best performance is if you don't swap at all. Avoid swapping, don't > relay on "fast swap". There's no such thing as "fast swap" ;-) > > Anyhow, theoretically when you read from RAID1, you read different pages > from different drives in parallel. That's why RAID1 theoretically has > two times faster reads than single drive. Writing to RAID1 is > theoretically the same speed as writing to single drive. Hi Aleksandar, Yes, you are right. I had forgotten about that. Just having a blonde day yesterday, I guess. Since swapin performance is more important than swapout, I really do have the best of both worlds the way things are set up. When it comes to swap, I'm a big believer that swap is a good thing. I've been reviewing this discussion on lkml: http://kerneltrap.org/node/3000/13875 My thinking is squarely in the Andrew Morton camp: ------ """Swapout is good. It frees up unused memory. I run my desktop machines at swappiness=100.""" and """My point is that decreasing the tendency of the kernel to swap stuff out is wrong. You really don't want hundreds of megabytes of BloatyApp's untouched memory floating about in the machine. Get it out on the disk, use the memory for something useful.""" ------ Though if the maximum latency muust be kept below a certain value, I can see where one might want to completely avoid swap. Of course, if a machine has *so* much memory that it *can't* use all available memory. i.e. the memory used by apps + the total amount of data read from disk <= physical memory, that would indeed be optimal. What I have is: 2 Pentium 4 Xeons. 3.2Ghz, Hyperthreaded, 2MB L2 per processor 4GB physical memory 2 250GB SATA drives on separate SATA channels running 40 Gnome desktops + 100 instances of a character based point of sale and accounting app. Plus samba file server, lightly loaded database server, lightly loaded intranet web server/Ruby on Rails app server. Typically, I have about 300MB in swap with very little swapin occurring and about 700MB cache, 50mb buffers. But as I've been told by one employee that they have noticed that one of the major incentives for other employees to choose to switch from their Windows desktop to a Linux desktop (via XDMCP) is the greatly improved speed, I'm interested in keeping it optimal as the load increases. I'm considering running the swappiness value higher. But I do start seeing some significant swapin at swappiness=100. (Typically, from 0 - 50 pages /sec averaged over 10 minutes according to systat/sar. That was what got me to thinking about how I had set up the swap. However, even at 50 p/s, that only represents 200k/s i/o which seems pretty trivial. I'm going to try a full day at swappiness=100 today and see how things look. I'll be NX'd into the machine my self doing some development work, which is about the best metric I can come up with. It's so very hard to come up with good solid metrics when it comes to this kind of tuning. -Steve