Re: Block device strategy and requests

[Jens Axboe]

On Thu, Apr 26 2001, Malcolm Beattie wrote:
> I'm designing a block device driver for a high performance disk
> subsystem with unusual characteristics. To what extent is the
> limited number of "struct request"s (128 by default) necessary for
> back-pressure? With this I/O subsystem it would be possible for the

Not at all

> strategy function to rip the requests from the request list straight
> away, arrange for the I/Os to be done to/from the buffer_heads (with
> no additional state required) with no memory "leak". This would
> effectively mean that the only limit on the number of I/Os queued
> would be the number of buffer_heads allocated; not a fixed number of
> "struct request"s in flight. Is this reasonable or does any memory or
> resource balancing depend on the number of I/Os outstanding being
> bounded?

The requests need not be bounded, as long as the buffer_heads are. I
don't see how the above scheme differs from some of the drivers that are
currently in the tree though?

> Also, there is a lot of flexibility in how often interrupts are sent
> to mark the buffer_heads up-to-date. (With the requests pulled
> straight off the queue, the job of end_that_request_first() in doing
> the linked list updates and bh->b_end_io() callbacks would be done by
> the interrupt routine directly.) At one extreme, I could take an
> interrupt for each 4K block issued and mark it up-to-date very
> quickly making for very low-latency I/O but a very large interrupt
> rate when I/O throughput is high. The alternative would be to arrange
> for an interrupt every n buffer_heads (or based on some other
> criterion) and only take an interrupt and mark buffers up-to-date on
> each of those). Are there any rules of thumb on which is best or
> doesn't it matter too much?

An interrupt per request would give you anywhere between 4kB and XXXkB
transfer per interrupt, depending on what you set your max_sectors to.
Going bigger than that probably won't make a whole lot of sense, and you
would have to do additional foot-work to make it happen. In theory. Only
real-life testing can tell you for sure, and how big requests you get
depends heavily on the workload so there will be no one true answer for
this.

-- 
Jens Axboe