Messages in this thread | | | Date | Tue, 15 Jul 2003 07:26:40 +0200 | From | Jens Axboe <> | Subject | Re: RFC on io-stalls patch |
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On Mon, Jul 14 2003, Andrea Arcangeli wrote: > On Mon, Jul 14, 2003 at 09:51:39PM +0200, Jens Axboe wrote: > > - rl = &q->rq; > > - if (!list_empty(&rl->free) && !blk_oversized_queue(q)) { > > + if ((rw == WRITE) && (blk_oversized_queue(q) || (rl->count < 4))) > > did you disable the oversized queue check completely for reads? This
Yes
> looks unsafe, you can end with loads of ram locked up this way, the > request queue cannot be limited in requests anymore. this isn't the > "request reservation", this a "nearly unlimited amount of ram locked in > for reads".
Sorry, but I think that is nonsense. This is the way we have always worked. You just have to maintain a decent queue length still (like we always have in 2.4) and there are no problems.
> Of course, the more reads can be in the queue, the less the background > write loads will hurt parallel apps like a kernel compile as shown in > xtar_load. > > This is very different from the schedule advantage provided by the old > queue model. If you allow an unlimited I/O queue for reads, that means > the I/O queues will be filled by an huge amount of reads and a few > writes (no matter how fast the xtar_load is writing to disk). > > In the past (2.4.22pre4) the I/O queue would been at most 50/50, with > your patch it can be 90/10, hence it can generate an huge performance > difference, that can penealize tremendously the writers in server loads > using fsync plus it can hurt the VM badly if all ram is locked up by > parallel reads. Of course contest mostly cares about reads, not writes. > > Overall I think your patch is unsafe and shouldn't be applied.
It is _not_ unsafe, stop spewing nonsense like that. The patch should not be applied, it's just the first few things I did to see if it would make a difference like I described. And it had a big effect, so I posted results and went to bed. Know we have a grounds for further discussion, and I'll bench the changes seperately too as well. It's about getting data points you can use, you have to try extremese as well.
> Still if you want to allow 50/50, go ahead, that logic in pre4 was an > order of magnitude more fair and generic than this patch.
Sigh... No I don't want 90/10 distribution of course, that would be silly.
-- Jens Axboe
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