On Thu, Jan 07, 2021 at 09:21:11AM +1100, Dave Chinner wrote: > On Wed, Jan 06, 2021 at 04:45:48PM +0800, Ming Lei wrote: > > On Tue, Jan 05, 2021 at 07:39:38PM +0100, Christoph Hellwig wrote: > > > At least for iomap I think this is the wrong approach. Between the > > > iomap and writeback_control we know the maximum size of the writeback > > > request and can just use that. > > > > I think writeback_control can tell us nothing about max pages in single > > bio: > > By definition, the iomap tells us exactly how big the IO is going to > be. i.e. an iomap spans a single contiguous range that we are going > to issue IO on. Hence we can use that to size the bio exactly > right for direct IO. When I trace wpc->iomap.length in iomap_add_to_ioend() on the following fio randwrite/write, the length is 1GB most of times, maybe because it is one fresh XFS. fio --size=1G --bsrange=4k-4k --runtime=30 --numjobs=2 --ioengine=psync --iodepth=32 \ --directory=$DIR --group_reporting=1 --unlink=0 --direct=0 --fsync=0 --name=f1 \ --stonewall --rw=$RW sync Another reason is that pages in the range may be contiguous physically, so lots of pages may share one single bvec. > > > - wbc->nr_to_write controls how many pages to writeback, this pages > > usually don't belong to same bio. Also this number is often much > > bigger than BIO_MAX_PAGES. > > > > - wbc->range_start/range_end is similar too, which is often much more > > bigger than BIO_MAX_PAGES. > > > > Also page/blocks_in_page can be mapped to different extent too, which is > > only available when wpc->ops->map_blocks() is returned, > > We only allocate the bio -after- calling ->map_blocks() to obtain > the iomap for the given writeback range request. Hence we > already know how large the BIO could be before we allocate it. > > > which looks not > > different with mpage_writepages(), in which bio is allocated with > > BIO_MAX_PAGES vecs too. > > __mpage_writepage() only maps a page at a time, so it can't tell > ahead of time how big the bio is going to need to be as it doesn't > return/cache a contiguous extent range. So it's actually very > different to the iomap writeback code, and effectively does require > a BIO_MAX_PAGES vecs allocation all the time... > > > Or you mean we can use iomap->length for this purpose? But iomap->length > > still is still too big in case of xfs. > > if we are doing small random writeback into large extents (i.e. > iomap->length is large), then it is trivial to detect that we are > doing random writes rather than sequential writes by checking if the > current page is sequential to the last sector in the current bio. > We already do this non-sequential IO checking to determine if a new > bio needs to be allocated in iomap_can_add_to_ioend(), and we also > know how large the current contiguous range mapped into the current > bio chain is (ioend->io_size). Hence we've got everything we need to > determine whether we should do a large or small bio vec allocation > in the iomap writeback path... page->index should tell us if the workload is random or sequential, however still not easy to decide how many pages there will be in the next bio when iomap->length is large. Thanks, Ming
