On Thu, 11 Jan 2007, Andrew Morton wrote: > On Thu, 11 Jan 2007 03:04:00 -0800 (PST) > dean gaudet <[EMAIL PROTECTED]> wrote: > > > On Tue, 9 Jan 2007, Neil Brown wrote: > > > > > Imagine a machine with lots of memory - say 100Gig. > > > > i've had these problems on machines as "small" as 8GiB. the real problem > > is that the kernel will let millions of potential (write) IO ops stack up > > for a device which can handle only mere 100s of IOs per second. (and i'm > > not convinced it does the IOs in a sane order when it has millions to > > choose from) > > > > replacing the percentage based dirty_ratio / dirty_background_ratio with > > sane kibibyte units is a good fix... but i'm not sure it's sufficient. > > > > it seems like the "flow control" mechanism (i.e. dirty_ratio) should be on > > a device basis... > > > > try running doug ledford'd memtest.sh on an 8GiB box with a single disk, > > let it go a few minutes then ^C and type "sync". i've had to wait 10 > > minutes (2.6.18 with default vm settings). > > > > it makes it hard to guarantee a box can shutdown quickly -- nasty for > > setting up UPS on-battery timeouts for example. > > > > Increasing the request queue size should help there > (/sys/block/sda/queue/nr_requests). Maybe 25% or more benefit with that > test, at a guess.
hmm i've never had much luck with increasing nr_requests... if i get a chance i'll reproduce the problem and try that. > Probably initscripts should do that rather than leaving the kernel defaults > in place. It's a bit tricky for the kernel to do because the decision > depends upon the number of disks in the system, as well as the amount of > memory. > > Or perhaps the kernel should implement a system-wide limit on the number of > requests in flight. While avoiding per-device starvation. Tricky. actually a global dirty_ratio causes interference between devices which should otherwise not block each other... if you set up a "dd if=/dev/zero of=/dev/sdb bs=1M" it shouldn't affect write performance on sda -- but it does... because the dd basically dirties all of the "dirty_background_ratio" pages and then any task writing to sda has to block in the foreground... (i've had this happen in practice -- my hack fix is oflag=direct on the dd... but the problem still exists.) i'm not saying fixing any of this is easy, i'm just being a user griping about it :) -dean - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
