Jens Axboe <axboe@xxxxxxxxx> writes: > On 2012-12-12 11:11, Jan Kara wrote: >> On Wed 12-12-12 10:55:15, Shaohua Li wrote: >>> 2012/12/11 Jan Kara <jack@xxxxxxx>: >>>> Hi, >>>> >>>> I was looking into IO starvation problems where streaming sync writes (in >>>> my case from kjournald but DIO would look the same) starve reads. This is >>>> because reads happen in small chunks and until a request completes we don't >>>> start reading further (reader reads lots of small files) while writers have >>>> plenty of big requests to submit. Both processes end up fighting for IO >>>> requests and writer writes nr_batching 512 KB requests while reader reads >>>> just one 4 KB request or so. Here the effect is magnified by the fact that >>>> the drive has relatively big queue depth so it usually takes longer than >>>> BLK_BATCH_TIME to complete the read request. The net result is it takes >>>> close to two minutes to read files that can be read under a second without >>>> writer load. Without the big drive's queue depth, results are not ideal but >>>> they are bearable - it takes about 20 seconds to do the reading. And for >>>> comparison, when writer and reader are not competing for IO requests (as it >>>> happens when writes are submitted as async), it takes about 2 seconds to >>>> complete reading. >>>> >>>> Simple reproducer is: >>>> >>>> echo 3 >/proc/sys/vm/drop_caches >>>> dd if=/dev/zero of=/tmp/f bs=1M count=10000 & >>>> sleep 30 >>>> time cat /etc/* 2>&1 >/dev/null >>>> killall dd >>>> rm /tmp/f >>>> >>>> The question is how can we fix this? Two quick hacks that come to my mind >>>> are remove timeout from the batching logic (is it that important?) or >>>> further separate request allocation logic so that reads have their own >>>> request pool. More systematic fix would be to change request allocation >>>> logic to always allow at least a fixed number of requests per IOC. What do >>>> people think about this? >>> >>> As long as queue depth > workload iodepth, there is little we can do >>> to prioritize tasks/IOC. Because throttling a task/IOC means queue >>> will be idle. We don't want to idle a queue (especially for SSD), so >>> we always push as more requests as possible to the queue, which >>> will break any prioritization. As far as I know we always have such >>> issue in CFQ for big queue depth disk. >> Yes, I understand that. But actually big queue depth on its own doesn't >> make the problem really bad (at least for me). When the reader doesn't have >> to wait for free IO requests, it progresses at a reasonable speed. What >> makes it really bad is that big queue depth effectively disallows any use >> of ioc_batching() mode for the reader and thus it blocks in request >> allocation for every single read request unlike writer which always uses >> its full batch (32 requests). > > 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. 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. Cheers, Jeff -- 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
