Re: [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO

Matthew Wilcox
2024-02-27T15:39:45Z

On Tue, Feb 27, 2024 at 02:21:59AM -0500, Kent Overstreet wrote:
> On Mon, Feb 26, 2024 at 03:48:35PM -0800, Linus Torvalds wrote:
> > On Mon, 26 Feb 2024 at 14:46, Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > I really haven't tested this AT ALL. I'm much too scared.
> >
> > "Courage is not the absence of fear, but acting in spite of it"
> > - Paddington Bear / Michal Scott
> >
> > It seems to actually boot here.
> >
> > That said, from a quick test with lots of threads all hammering on the
> > same page - I'm still not entirely convinced it makes a difference.
> > Sure, the kernel profile changes, but filemap_get_read_batch() wasn't
> > very high up in the profile to begin with.
> >
> > I didn't do any actual performance testing, I just did a 64-byte pread
> > at offset 0 in a loop in 64 threads on my 32c/64t machine.
>
> Only rough testing, but this is looking like around a 25% performance
> increase doing 4k random reads on a 1G file with fio, 8 jobs, on my
> Ryzen 5950x - 16.7M -> 21.4M iops, very roughly. fio's a pig and we're
> only spending half our cpu time in the kernel, so the buffered read path
> is actually getting 40% or 50% faster.

Linus' patch only kicks in for 128 bytes or smaller. So what are you
measuring?