On Tue, May 30, 2023 at 12:31 PM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > On Mon, May 29, 2023 at 10:06:26PM -0400, chenzhiyin wrote: > > In the syscall test of UnixBench, performance regression occurred > > due to false sharing. > > > > The lock and atomic members, including file::f_lock, file::f_count > > and file::f_pos_lock are highly contended and frequently updated > > in the high-concurrency test scenarios. perf c2c indentified one > > affected read access, file::f_op. > > To prevent false sharing, the layout of file struct is changed as > > following > > (A) f_lock, f_count and f_pos_lock are put together to share the > > same cache line. > > (B) The read mostly members, including f_path, f_inode, f_op are > > put into a separate cache line. > > (C) f_mode is put together with f_count, since they are used > > frequently at the same time. > > > > The optimization has been validated in the syscall test of > > UnixBench. performance gain is 30~50%, when the number of parallel > > jobs is 16. > > > > Signed-off-by: chenzhiyin <zhiyin.chen@xxxxxxxxx> > > --- > > Sounds interesting, but can we see the actual numbers, please? > So struct file is marked with __randomize_layout which seems to make > this whole reordering pointless or at least only useful if the > structure randomization Kconfig is turned off. Is there any precedence > to optimizing structures that are marked as randomizable? Good question! Also does the impressive improvement is gained only with (A)+(B)+(C)? (A) and (B) make sense, but something about the claim (C) does not sit right. Can you explain this claim? Putting the read mostly f_mode with frequently updated f_count seems counter to the goal of your patch. Aren't f_mode and f_flags just as frequently accessed as f_op? Shouldn't f_mode belong with the read-mostly members? What am I missing? Thanks, Amir.
