Re: [PATCH] deadline io scheduler

Daniel Pittman
2002-09-26T23:18:30Z

On Thu, 26 Sep 2002, Jens Axboe wrote:
> On Thu, Sep 26 2002, Daniel Pittman wrote:
>> On Thu, 26 Sep 2002, Jens Axboe wrote:
>> > On Wed, Sep 25 2002, Andrew Morton wrote:
>>
>> [...]
>>
>> > writes_starved. This controls how many times reads get preferred
>> > over writes. The default is 2, which means that we can serve two
>> > batches of reads over one write batch. A value of 4 would mean that
>> > reads could skip ahead of writes 4 times. A value of 1 would give
>> > you 1:1 read:write, ie no read preference. A silly value of 0 would
>> > give you write preference, always.
>>
>> Actually, a value of zero doesn't sound completely silly to me, right
>> now, since I have been doing a lot of thinking about video capture
>> recently.
>>
>> How much is it going to hurt a filesystem like ext[23] if that value
>> is set to zero while doing large streaming writes -- something like
>> (almost) uncompressed video at ten to twenty meg a second, for
>> gigabytes?
>
> You are going to stalll all reads indefinately :-)

Which has some potentially fatal consequences, really, if any of the
capture code gets paged out before the streaming write starts, or if the
filesystem needs to read a bitmap block or so, as Rik points out.

>> This is a situation where, for a dedicated machine, delaying reads
>> almost forever is actually a valuable thing. At least, valuable until
>> it stops the writes from being able to proceed.
>
> Well 0 should achieve that quite fine

Would you consider allowing something akin to 'writes_starved = -4' to
allow writes to bypass reads only 4 times -- a preference for writes,
but not forever?

That's going to express the bias I (think I) want for this case, but
it's not going to be able to stall a read forever...

Daniel

--
It is quite humbling to realize that the storage occupied by the longest line
from a typical Usenet posting is sufficient to provide a state space so vast
that all the computation power in the world can not conquer it.
-- Dave Wallace