Re: large pause when opening file descriptor which is power of 2

On Wed, Apr 26, 2023 at 08:46:28PM +0100, Al Viro wrote:
> On Wed, Apr 26, 2023 at 08:13:37PM +0100, Matthew Wilcox wrote:
> > On Wed, Apr 26, 2023 at 05:58:06PM +0000, Kernel.org Bugbot wrote:
> > > When running a threaded program, and opening a file descriptor that
> > > is a power of 2 (starting at 64), the call takes a very long time to
> > > complete. Normally such a call takes less than 2us. However with this
> > > issue, I've seen the call take up to around 50ms. Additionally this only
> > > happens the first time, and not subsequent times that file descriptor is
> > > used. I'm guessing there might be some expansion of some internal data
> > > structures going on. But I cannot see why this process would take so long.
> > 
> > Because we allocate a new block of memory and then memcpy() the old
> > block of memory into it.  This isn't surprising behaviour to me.
> > I don't think there's much we can do to change it (Allocating a
> > segmented array of file descriptors has previously been vetoed by
> > people who have programs with a million file descriptors).  Is it
> > causing you problems?
> 
> FWIW, I suspect that this is not so much allocation + memcpy.
>         /* make sure all fd_install() have seen resize_in_progress
> 	 * or have finished their rcu_read_lock_sched() section.
> 	 */
> 	if (atomic_read(&files->count) > 1)
> 		synchronize_rcu();
> 
> in expand_fdtable() is a likelier source of delays.

Perhaps?  The delay seemed to be roughly doubling with the test program,
so I assumed it was primarily the memcpy() cost for the reporter's
system:

FD=64 duration=12565293
FD=128 duration=24755063
FD=256 duration=7602777

... although now I've pasted it, I see my brain skipped one digit, so
256 was faster than 64, not about twice as slow as 128.