On Fri, Mar 03, 2017 at 02:43:49PM +1100, Neil Brown wrote: > On Fri, Feb 17 2017, Shaohua Li wrote: > > > Previous patch (raid5: only dispatch IO from raid5d for harddisk raid) > > defers IO dispatching. The goal is to create better IO pattern. At that > > time, we don't sort the deffered IO and hope the block layer can do IO > > merge and sort. Now the raid5-cache writeback could create large amount > > of bios. And if we enable muti-thread for stripe handling, we can't > > control when to dispatch IO to raid disks. In a lot of time, we are > > dispatching IO which block layer can't do merge effectively. > > > > This patch moves further for the IO dispatching defer. We accumulate > > bios, but we don't dispatch all the bios after a threshold is met. This > > 'dispatch partial portion of bios' stragety allows bios coming in a > > large time window are sent to disks together. At the dispatching time, > > there is large chance the block layer can merge the bios. To make this > > more effective, we dispatch IO in ascending order. This increases > > request merge chance and reduces disk seek. > > I can see the benefit of batching and sorting requests. > > I wonder if the extra complexity of grouping together 512 requests, then > submitting the "first" 128 is really worth it. Have you measured the > value of that? I'm pretty sure I tried. The whole point of dispatching the first 128 is we don't have a better pipeline. Grouping 512 and then dispatching them together definitely improve the IO patter, but the request accumulation takes time, we will have no IO running in the window. > If you just submitted every time you got 512 requests, you could use > list_sort() on the bio list and wouldn't need an array. > > If an array really is best, it would be really nice if "sort" could pass > a 'void*' down to the cmp function, > and it could sort all bios that are > *after* last_bio_pos first, and then the others. That would make the > code much simpler. I guess sort() could be changed (list_sort() already > has a 'priv' argument like this). Ok, I'll change this to a list. And add extra pointer to record the last sorted entry. I didn't see the sort uses much time in my profile, but the merge sort looks better. Will do the change. > If we cannot change sort(), then maybe use lib/bsearch.c for the binary > search. Performing two comparisons in the loop of a binary search > should get a *fail* in any algorithms class!! > The "pending_data" array that you have added to the r5conf structure > adds 4096 bytes. This means it is larger than a page, which is best > avoided (though it is unlikely to cause problems). I would allocate it > separately. Yep, already fixed internally. > > So there is a lot that I don't really like, but it seems like a good > idea in principle. ok, thanks for your time! Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
