Messages in this thread | | | Date | Thu, 22 Feb 2001 10:59:20 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: ll_rw_block/submit_bh and request limits |
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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
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