Messages in this thread | | | Subject | Re: [PATCH v4] mm: don't cap request size based on read-ahead setting | From | Jens Axboe <> | Date | Fri, 18 Nov 2016 12:34:18 -0700 |
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On 11/18/2016 11:02 AM, Johannes Weiner wrote: > On Thu, Nov 17, 2016 at 02:23:10PM -0700, Jens Axboe wrote: >> We ran into a funky issue, where someone doing 256K buffered reads saw >> 128K requests at the device level. Turns out it is read-ahead capping >> the request size, since we use 128K as the default setting. This doesn't >> make a lot of sense - if someone is issuing 256K reads, they should see >> 256K reads, regardless of the read-ahead setting, if the underlying >> device can support a 256K read in a single command. >> >> To make matters more confusing, there's an odd interaction with the >> fadvise hint setting. If we tell the kernel we're doing sequential IO on >> this file descriptor, we can get twice the read-ahead size. But if we >> tell the kernel that we are doing random IO, hence disabling read-ahead, >> we do get nice 256K requests at the lower level. This is because >> ondemand and forced read-ahead behave differently, with the latter doing >> the right thing. An application developer will be, rightfully, >> scratching his head at this point, wondering wtf is going on. A good one >> will dive into the kernel source, and silently weep. > > With the FADV_RANDOM part of the changelog updated, this looks good to > me. Just a few nitpicks below. > >> This patch introduces a bdi hint, io_pages. This is the soft max IO size >> for the lower level, I've hooked it up to the bdev settings here. >> Read-ahead is modified to issue the maximum of the user request size, >> and the read-ahead max size, but capped to the max request size on the >> device side. The latter is done to avoid reading ahead too much, if the >> application asks for a huge read. With this patch, the kernel behaves >> like the application expects. >> >> Signed-off-by: Jens Axboe <axboe@fb.com> > >> @@ -207,12 +207,17 @@ int __do_page_cache_readahead(struct address_space >> *mapping, struct file *filp, >> * memory at once. >> */ >> int force_page_cache_readahead(struct address_space *mapping, struct file >> *filp, > > Linewrap (but you already knew that ;))
Yeah, dunno wtf happened there...
>> - pgoff_t offset, unsigned long nr_to_read) >> + pgoff_t offset, unsigned long nr_to_read) >> { >> + struct backing_dev_info *bdi = inode_to_bdi(mapping->host); >> + struct file_ra_state *ra = &filp->f_ra; >> + unsigned long max_pages; >> + >> if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages)) >> return -EINVAL; >> >> - nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages); >> + max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages); >> + nr_to_read = min(nr_to_read, max_pages); > > It would be useful to have the comment on not capping below optimal IO > size from ondemand_readahead() here as well.
Good idea, I'll copy/paste the comment from the ondemand part.
>> @@ -369,10 +374,18 @@ ondemand_readahead(struct address_space *mapping, >> bool hit_readahead_marker, pgoff_t offset, >> unsigned long req_size) >> { >> - unsigned long max = ra->ra_pages; >> + struct backing_dev_info *bdi = inode_to_bdi(mapping->host); >> + unsigned long max_pages = ra->ra_pages; >> pgoff_t prev_offset; >> >> /* >> + * If the request exceeds the readahead window, allow the read to >> + * be up to the optimal hardware IO size >> + */ >> + if (req_size > max_pages && bdi->io_pages > max_pages) >> + max_pages = min(req_size, bdi->io_pages); >> + >> + /* >> * start of file >> */ >> if (!offset) > > Please feel free to add: > > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Thanks!
-- Jens Axboe
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