Re: Linux 4.15-rc6

[Linus Torvalds]

On Tue, Jan 2, 2018 at 12:28 PM, Andres Freund <andres@xxxxxxxxxxx> wrote:
>
> I thought it'd be interesting to run a short benchmark to be able to
> estimate the impact of the PTI work on postgres workloads (which I work
> on). On my skylake laptop, a memory resident, OLTP workload with 16
> connections results in:

Yeah, that's actually pretty much in line with expectations.

Something around 5% performance impact of the isolation is what people
are looking at.

Obviously it depends on just exactly what you do. Some loads will
hardly be affected at all, if they just spend all their time in user
space. And if you do a lot of small system calls, you might see
double-digit slowdowns.

> This isn't a complaint, I just thought it might be useful
> information. If it helps for anything/anybody, I'm happy to run
> additional benchmarks / provide additional information.

Note that it will depend heavily on the hardware too. Older CPU's
without PCID will be impacted more by the isolation. And I think some
of the back-ports won't take advantage of PCID even on newer hardware.

              Linus