On Mon, Aug 14, 2017 at 09:14:03AM -0600, Jens Axboe wrote: > On 08/14/2017 09:06 AM, Minchan Kim wrote: > > On Mon, Aug 14, 2017 at 08:36:00AM -0600, Jens Axboe wrote: > >> On 08/14/2017 02:50 AM, Minchan Kim wrote: > >>> Hi Jens, > >>> > >>> On Fri, Aug 11, 2017 at 08:26:59AM -0600, Jens Axboe wrote: > >>>> On 08/11/2017 04:46 AM, Christoph Hellwig wrote: > >>>>> On Wed, Aug 09, 2017 at 08:06:24PM -0700, Dan Williams wrote: > >>>>>> I like it, but do you think we should switch to sbvec[<constant>] to > >>>>>> preclude pathological cases where nr_pages is large? > >>>>> > >>>>> Yes, please. > >>>>> > >>>>> Then I'd like to see that the on-stack bio even matters for > >>>>> mpage_readpage / mpage_writepage. Compared to all the buffer head > >>>>> overhead the bio allocation should not actually matter in practice. > >>>> > >>>> I'm skeptical for that path, too. I also wonder how far we could go > >>>> with just doing a per-cpu bio recycling facility, to reduce the cost > >>>> of having to allocate a bio. The on-stack bio parts are fine for > >>>> simple use case, where simple means that the patch just special > >>>> cases the allocation, and doesn't have to change much else. > >>>> > >>>> I had a patch for bio recycling and batched freeing a year or two > >>>> ago, I'll see if I can find and resurrect it. > >>> > >>> So, you want to go with per-cpu bio recycling approach to > >>> remove rw_page? > >>> > >>> So, do you want me to hold this patchset? > >> > >> I don't want to hold this series up, but I do think the recycling is > >> a cleaner approach since we don't need to special case anything. I > >> hope I'll get some time to dust it off, retest, and post soon. > > > > I don't know how your bio recycling works. But my worry when I heard > > per-cpu bio recycling firstly is if it's not reserved pool for > > BDI_CAP_SYNCHRONOUS(IOW, if it is shared by several storages), > > BIOs can be consumed by slow device(e.g., eMMC) so that a bio for > > fastest device(e.g., zram in embedded system) in the system can be > > stucked to wait on bio until IO for slow deivce is completed. > > > > I guess it would be a not rare case for swap device under severe > > memory pressure because lots of page cache are already reclaimed when > > anonymous page start to be reclaimed so that many BIOs can be consumed > > for eMMC to fetch code but swap IO to fetch heap data would be stucked > > although zram-swap is much faster than eMMC. > > As well, time to wait to get BIO among even fastest devices is > > simple waste, I guess. > > I don't think that's a valid concern. First of all, for the recycling, > it's not like you get to wait on someone else using a recycled bio, > if it's not there you simply go to the regular bio allocator. There > is no waiting for free. The idea is to have allocation be faster since > we can avoid going to the memory allocator for most cases, and speed > up freeing as well, since we can do that in batches too. I doubt how it performs well because at the beginning of this thread[1], Ross said that with even dynamic bio allocation without rw_page, there is no regression in some testing. [1] http://lkml.kernel.org/r/<20170728165604.10455-1-ross.zwisler@xxxxxxxxxxxxxxx> > > Secondly, generally you don't have slow devices and fast devices > intermingled when running workloads. That's the rare case. Not true. zRam is really popular swap for embedded devices where one of low cost product has a really poor slow nand compared to lz4/lzo [de]comression. > > > To me, bio suggested by Christoph Hellwig isn't diverge current > > path a lot and simple enough to change. > > It doesn't diverge it a lot, but it does split it up. > > > Anyway, I'm okay with either way if we can remove rw_page without > > any regression because the maintainance of both rw_page and > > make_request is rather burden for zram, too. > > Agree, the ultimate goal of both is to eliminate the need for the > rw_page hack. Yeb. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
