On Fri, Apr 5, 2019 at 12:17 AM Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Thu, Apr 04, 2019 at 07:57:37PM +0300, Amir Goldstein wrote: > > This patch improves performance of mixed random rw workload > > on xfs without relaxing the atomic buffered read/write guaranty > > that xfs has always provided. > > > > We achieve that by calling generic_file_read_iter() twice. > > Once with a discard iterator to warm up page cache before taking > > the shared ilock and once again under shared ilock. > > This will race with thing like truncate, hole punching, etc that > serialise IO and invalidate the page cache for data integrity > reasons under the IOLOCK. These rely on there being no IO to the > inode in progress at all to work correctly, which this patch > violates. IOWs, while this is fast, it is not safe and so not a > viable approach to solving the problem. > This statement leaves me wondering, if ext4 does not takes i_rwsem on generic_file_read_iter(), how does ext4 (or any other fs for that matter) guaranty buffered read synchronization with truncate, hole punching etc? The answer in ext4 case is i_mmap_sem, which is read locked in the page fault handler. And xfs does the same type of synchronization with MMAPLOCK, so while my patch may not be safe, I cannot follow why from your explanation, so please explain if I am missing something. One thing that Darrick mentioned earlier was that IOLOCK is also used by xfs to synchronization pNFS leases (probably listed under 'etc' in your explanation). I consent that my patch does not look safe w.r.t pNFS leases, but that can be sorted out with a hammer #ifndef CONFIG_EXPORTFS_BLOCK_OPS or with finer instruments. > FYI, I'm working on a range lock implementation that should both > solve the performance issue and the reader starvation issue at the > same time by allowing concurrent buffered reads and writes to > different file ranges. > > IO range locks will allow proper exclusion for other extent > manipulation operations like fallocate and truncate, and eventually > even allow truncate, hole punch, file extension, etc to run > concurrently with other non-overlapping IO. They solve more than > just the performance issue you are seeing.... > I'm glad to hear that. IO range locks are definitely a more wholesome solution to the problem looking forward. However, I am still interested to continue the discussion on my POC patch. One reason is that I am guessing it would be much easier for distros to backport and pick up to solve performance issues. Even if my patch doesn't get applied upstream nor picked by distros, I would still like to understand its flaws and limitations. I know... if I break it, I get to keep the pieces, but the information that you provide helps me make my risk assessments. Thanks, Amir.
