[email protected] wrote: > > 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.
Which flag, on which function ? At the end of the day, aren't all functions reading from disk mapping to an read(3) ? >> 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. I agree with you, but my understanding is that with a high value for swappiness the kernel will swap out processes in order to make space for the file system cache. Look at this test: http://lwn.net/Articles/100978/ Just doing dd's they get the vm to swap memory out, which confirms my understanding of it. If I am right, then to obtain the result you are talking about ("the kernel won't throw away memory unless something else is asking for it"), you need to set swappiness to zero. > >> 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. This is the latest (2.6.31) kernel from L. Torvalds, and it still has swappiness=60 http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=mm/vmscan.c;h=94e86dd6954c295830478011fd8e71465f1a9f2d;hb=e07cccf4046978df10f2e13fe2b99b2f9b3a65db#l127 So which distribution do you use (I use Fedora 10, Ubuntu 9.10 and CentOS 5.3, they leave the default of 60) ? What value to they put by default for swappiness ? Anyway, I have been setting swappiness to zero by default on all the systems I take care of for a few years, so if there is a reason why I should not, I'd love to hear it. Why do you set it at a value different than zero (what is the expected outcome, how is it different than if it were set at zero) ? If neither 0 nor 60 are the right values, what is the right value ? If it depends on the load, how do you make an objective decision ? Thanks. -- 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/
