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

Kent Overstreet
2024-02-27T16:47:24Z

On Tue, Feb 27, 2024 at 08:34:39AM -0800, Linus Torvalds wrote:
> On Mon, 26 Feb 2024 at 23:22, Kent Overstreet <kent.overstreet@linux.dev> wrote:
> >
> > 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.
> >
> > So I'd say that's substantial.
>
> No, you're doing something wrong. The new fastread logic only
> triggers for reads <= 128 bytes, so you must have done some other
> major change (like built a kernel without the mitigations, and
> compared it to one with mitigations - that would easily be 25%
> depending on hardware).

Like I replied to willy - the "4k" was a typo from force of habit, I was
doing 64 byte random reads.