On Thu 13-12-12 14:30:42, Jens Axboe wrote: > On 2012-12-12 20:41, Jeff Moyer wrote: > > Jeff Moyer <jmoyer@xxxxxxxxxx> writes: > > > >>> I agree. This isn't about scheduling, we haven't even reached that part > >>> yet. Back when we split the queues into read vs write, this problem > >>> obviously wasn't there. Now we have sync writes and reads, both eating > >>> from the same pool. The io scheduler can impact this a bit by forcing > >>> reads to must allocate (Jan, which io scheduler are you using?). CFQ > >>> does this when it's expecting a request from this process queue. > >>> > >>> Back in the day, we used to have one list. To avoid a similar problem, > >>> we reserved the top of the list for reads. With the batching, it's a bit > >>> more complicated. If we make the request allocation (just that, not the > >>> scheduling) be read vs write instead of sync vs async, then we have the > >>> same issue for sync vs buffered writes. > >>> > >>> How about something like the below? Due to the nature of sync reads, we > >>> should allow a much longer timeout. The batch is really tailored towards > >>> writes at the moment. Also shrink the batch count, 32 is pretty large... > >> > >> Does batching even make sense for dependent reads? I don't think it > >> does. > > > > Having just read the batching code in detail, I'd like to ammend this > > misguided comment. Batching logic kicks in when you happen to be lucky > > enough to use up the last request. As such, I'd be surprised if the > > patch you posted helped. Jens, don't you think the writer is way more > > likely to become the batcher? I do agree with shrinking the batch count > > to 16, whether or not the rest of the patch goes in. > > > >> Assuming you disagree, then you'll have to justify that fixed > >> time value of 2 seconds. The amount of time between dependent reads > >> will vary depending on other I/O sent to the device, the properties of > >> the device, the I/O scheduler, and so on. If you do stick 2 seconds in > >> there, please comment it. Maybe it's time we started keeping track of > >> worst case Q->C time? That could be used to tell worst case latency, > >> and adjust magic timeouts like this one. > >> > >> I'm still thinking about how we might solve this in a cleaner way. > > > > The way things stand today, you can do a complete end run around the I/O > > scheduler by queueing up enough I/O. To address that, I think we need > > to move to a request list per io_context as Jan had suggested. That > > way, we can keep the logic about who gets to submit I/O when in one > > place. > > > > Jens, what do you think? > > I think that is pretty extreme. We have way too much accounting around > this already, and I'd rather just limit the batching than make > per-ioc request lists too. > > I agree the batch addition isn't super useful for the reads. It really > is mostly a writer thing, and the timing reflects that. > > The problem is really that the WRITE_SYNC is (for Jan's case) behaving > like buffered writes, so it eats up a queue of requests very easily. On > the allocation side, the assumption is that WRITE_SYNC behaves like > dependent reads. Similar to a dd with oflag=direct, not like a flood of > requests. For dependent sync writes, our current behaviour is fine, we > treat them like reads. For commits of WRITE_SYNC, they should be treated > like async WRITE instead. Yeah. But it's similar to what happens when you run fsync() on a large dirty file. That will also submit a lot of WRITE_SYNC requests... kjournald could probably use WRITE instead of WRITE_SYNC for large commits. It's just that we don't really want to give e.g. DIO a preference over kjournald because transaction commit can effectively block any metadata changes on the filesystem. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
