On Fri, May 07, 2021 at 10:06:31AM -0400, Brian Foster wrote: > > <nod> So I guess I'm saying that my resistance to /this/ part of the > > changes is melting away. For a 2GB+ write IO, I guess the extra overhead > > of poking a workqueue can be amortized over the sheer number of pages. > > I think the main question is what is a suitable size threshold to kick > an ioend over to the workqueue? Looking back, I think this patch just > picked 256k randomly to propose the idea. ISTM there could be a > potentially large window from the point where I/O latency starts to > dominate (over the extra context switch for wq processing) and where the > softlockup warning thing will eventually trigger due to having too many > pages. I think that means we could probably use a more conservative > value, I'm just not sure what value should be (10MB, 100MB, 1GB?). If > you have a reproducer it might be interesting to experiment with that. To my mind, there are four main types of I/Os. 1. Small, dependent reads -- maybe reading a B-tree block so we can get the next pointer. Those need latency above all. 2. Readahead. Tend to be large I/Os and latency is not a concern 3. Page writeback which tend to be large and can afford the extra latency. 4. Log writes. These tend to be small, and I'm not sure what increasing their latency would do. Probably bad. I like 256kB as a threshold. I think I could get behind anything from 128kB to 1MB. I don't think playing with it is going to be really interesting because most IOs are going to be far below or far above that threshold.
