On Thu, Jun 13, 2019 at 03:13:40PM -0600, Andreas Dilger wrote: > There are definitely workloads that require multiple threads doing non-overlapping > writes to a single file in HPC. This is becoming an increasingly common problem > as the number of cores on a single client increase, since there is typically one > thread per core trying to write to a shared file. Using multiple files (one per > core) is possible, but that has file management issues for users when there are a > million cores running on the same job/file (obviously not on the same client node) > dumping data every hour. Mixed buffered and O_DIRECT though? That profile looks like just buffered IO to me. > We were just looking at this exact problem last week, and most of the threads are > spinning in grab_cache_page_nowait->add_to_page_cache_lru() and set_page_dirty() > when writing at 1.9GB/s when they could be writing at 5.8GB/s (when threads are > writing O_DIRECT instead of buffered). Flame graph is attached for 16-thread case, > but high-end systems today easily have 2-4x that many cores. Yeah I've been spending some time on buffered IO performance too - 4k page overhead is a killer. bcachefs has a buffered write path that looks up multiple pages at a time and locks them, and then copies the data to all the pages at once (I stole the idea from btrfs). It was a very significant performance increase. https://evilpiepirate.org/git/bcachefs.git/tree/fs/bcachefs/fs-io.c#n1498
