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Subject[patch(?)] question wrt context switching during disk i/o
Also sprach Mike Galbraith:
} On Fri, 20 Oct 2000, Mark Hahn wrote:
}
} > > This is something that has been bugging me for a while. I notice
} > > on my system that during disk write we do much context switching,
} > > but not during disk read. Why is that?
} >
} > bdflush is broken in current kernels. I posted to linux-mm about this,
} > but Rik et al haven't shown any interest. I normally see bursts of
} > up to around 40K cs/second when doing writes; I hacked a little
} > premption counter into the kernel and verified that they're practially
} > all bdflush...
}
There's some strangness in bdflush(). The comment says:

/*
* If there are still a lot of dirty buffers around,
* skip the sleep and flush some more. Otherwise, we
* go to sleep waiting a wakeup.
*/
if (!flushed || balance_dirty_state(NODEV) < 0) {
run_task_queue(&tq_disk);
schedule();
}

but the comment for balance_dirty_state() says:

/* -1 -> no need to flush
0 -> async flush
1 -> sync flush (wait for I/O completation) */
int balance_dirty_state(kdev_t dev)
{

Which leads me to believe that the `<' should be either `==' or `<='. I
tried it with the `<=' and it doesn't seem to be so bad...Here's a patch
to see if it helps you?

--
|| Bill Wendling wendling@ganymede.isdn.uiuc.edu
--- linux/fs/buffer.c Sat Oct 21 02:55:41 2000
+++ linux-2.4.0-test10pre4/fs/buffer.c Sat Oct 21 12:27:10 2000
@@ -2683,7 +2683,7 @@
* skip the sleep and flush some more. Otherwise, we
* go to sleep waiting a wakeup.
*/
- if (!flushed || balance_dirty_state(NODEV) < 0) {
+ if (!flushed || balance_dirty_state(NODEV) <= 0) {
run_task_queue(&tq_disk);
schedule();
}
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