Subject: Give kjournald a IOPRIO_CLASS_RT io priority From: Arjan van de Ven <[EMAIL PROTECTED]>
With latencytop, I noticed that the (in memory) atime updates during a kernel build had latencies of 600 msec or longer; this is obviously not so nice behavior. Other EXT3 journal related operations had similar or even longer latencies. Digging into this a bit more, it appears to be an interaction between EXT3 and CFQ in that CFQ tries to be fair to everyone, including kjournald. However, in reality, kjournald is "special" in that it does a lot of journal work and effectively this leads to a twisted kind of "mass priority inversion" type of behavior. The good news is that CFQ already has the infrastructure to make certain processes special... JBD just wasn't using that quite yet. The patch below makes kjournald of the IOPRIO_CLASS_RT priority to break this priority inversion behavior. With this patch, the latencies for atime updates (and similar operation) go down by a factor of 3x to 4x ! Signed-off-by: Arjan van de Ven <[EMAIL PROTECTED]> diff -purN linux-2.6.23-rc9.org/fs/jbd/journal.c linux-2.6.23-rc9.lt/fs/jbd/journal.c --- linux-2.6.23-rc9.org/fs/jbd/journal.c 2007-10-02 05:24:52.000000000 +0200 +++ linux-2.6.23-rc9.lt/fs/jbd/journal.c 2007-10-14 00:06:55.000000000 +0200 @@ -35,6 +35,7 @@ #include <linux/kthread.h> #include <linux/poison.h> #include <linux/proc_fs.h> +#include <linux/ioprio.h> #include <asm/uaccess.h> #include <asm/page.h> @@ -131,6 +132,8 @@ static int kjournald(void *arg) printk(KERN_INFO "kjournald starting. Commit interval %ld seconds\n", journal->j_commit_interval / HZ); + current->ioprio = (IOPRIO_CLASS_RT << IOPRIO_CLASS_SHIFT) | 4; + /* * And now, wait forever for commit wakeup events. */ - 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/