On Tue, May 30, 2023 at 06:55:49PM -0700, Eric Biggers wrote: > On Tue, May 30, 2023 at 10:50:42AM +0200, Christian Brauner 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? > > Most people don't use CONFIG_RANDSTRUCT. So it's still worth optimizing struct > layouts for everyone else. Ok, good to know. We should still see actual numbers and the commit message should mention that this interacts with __randomize_layout and why it's still useful.
