Re: pagecache locking (was: bcachefs status update) merged)

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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



[Index of Archives]     [XFS Filesystem Development (older mail)]     [Linux Filesystem Development]     [Linux Audio Users]     [Yosemite Trails]     [Linux Kernel]     [Linux RAID]     [Linux SCSI]


  Powered by Linux