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. -- Jens Axboe -- 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
