Re: ll_rw_block/submit_bh and request limits

[Linus Torvalds]

On Thu, 22 Feb 2001, Jens Axboe wrote:

> On Thu, Feb 22 2001, Marcelo Tosatti wrote:
> > The following piece of code in ll_rw_block() aims to limit the number of
> > locked buffers by making processes throttle on IO if the number of on
> > flight requests is bigger than a high watermaker. IO will only start
> > again if we're under a low watermark.
> > 
> >                 if (atomic_read(&queued_sectors) >= high_queued_sectors) {
> >                         run_task_queue(&tq_disk);
> >                         wait_event(blk_buffers_wait,
> >                         	atomic_read(&queued_sectors) < low_queued_sectors);
> >                 }
> > 
> > 
> > However, if submit_bh() is used to queue IO (which is used by ->readpage()
> > for ext2, for example), no throttling happens.
> > 
> > It looks like ll_rw_block() users (writes, metadata reads) can be starved
> > by submit_bh() (data reads). 
> > 
> > If I'm not missing something, the watermark check should be moved to
> > submit_bh(). 
> 
> We might as well put it there, the idea was to not lock this one
> buffer either but I doubt this would make any different in reality :-)

I'd prefer for this check to be a per-queue one.

Right now a slow device (like a floppy) would artifically throttle a fast
one, if I read the above right. So instead of moving it down the
call-chain, I'd rather remove the check completely as it looks wrong to
me.

Now, if people want throttling, I'd much rather see that done per-queue.

(There's another level of throttling that migth make sense: right now the
swap-out code has this "nr_async_pages" throttling which is very different
from the queue throttling. It might make sense to move that _VM_-level
throttling to writepage too - so that syncing of dirty mmap's will not
cause an overload of pages in flight. This was one of the reasons I
changed the semantics of write-page - so that shared mappings could do
that kind of smoothing too).

		Linus