Re: 2.2.19pre3 and poor reponse to RT-scheduled processes?

[torvalds@transmeta.com (Linus Torvalds)]

In article <20001230191639.E9332@athlon.random>,
Andrea Arcangeli  <andrea@suse.de> wrote:
>On Fri, Dec 29, 2000 at 04:54:23PM -0500, Rafal Boni wrote:
>> Now my box behaves much more reasonably... I'll just have to beat harder
>> on it and see what happens.
>
>Another thing: while writing to disk if you want low latency readers you can
>do:
>
>	elvtune -r 1 /dev/hd[abcd]
>
>The 1/2 seconds stalls you see could be just because of applications that waits
>I/O synchronously while the elevator is reodering I/O requests (and even if the
>elevator wouldn't reorder anything the new requests would go to the end of the
>I/O queue so they would have some higher latency anyways).

That sounds like too long a stall to be due to elevator ordering except
with some _really_ unlucky access patterns (or with slow disks). 

There are other, equally likely, candidates for these kinds of stalls:

 - filesystem locks. Especially the ext2 superblock lock. You can easily
   hit this one, as some ext2 functions actually do a lot of IO while
   holding the lock.

 - synchronously waiting for bdflush with balance_dirty_buffers().
   Especially mixed with the above.

A mixture of the two above will bascally stall the whole machine: almost
any non-cached file access ends up waiting for the superblock lock and
bdflush, and it can easily get quite unfair.

		Linus
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