linux-kernel - Re: Feedback on 4.9 performance after PTI fixes

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Date:   Tue, 9 Jan 2018 08:09:41 +0100
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Yves-Alexis Perez <corsac@...ian.org>
Subject: Re: Feedback on 4.9 performance after PTI fixes

Hi again,

updating the table after Yves-Alexis' comment on PCID. Rerunning the test
with -cpu=Haswell to enable PCID gave me much better numbers :

On Sun, Jan 07, 2018 at 11:18:56AM +0100, Willy Tarreau wrote:
> Hi,
> 
> I managed to take a bit of time to run some more tests on PTI both
> native and hosted in KVM, on stable versions built with
> CONFIG_PAGE_TABLE_ISOLATION=y. Here it's 4.9.75, used both on the
> host and the VM. I could compare pti=on/off both in the host and the
> VM. A single CPU was exposed in the VM.
> 
> It was running on my laptop (core i7 3320M at 2.6 GHz, 3.3 GHz single
> core turbo).
> 
> The test was run on haproxy's ability to forward connections. The
> results are below :
> 
>    Host  |  Guest  | conn/s  | ratio_to_host |  ratio_to_VM | Notes
> ---------+---------+---------+---------------+--------------+----------------
>  pti=off |   -     |  27400  |     100.0%    |      -       | host reference
>  pti=off | pti=off |  24200  |      88.3%    |    100.0%    | VM reference 
>  pti=off | pti=on  |  13300  |      48.5%    |     55.0%    |
>  pti=on  |   -     |  23800  |      86.9%    |      -	      | protected host
>  pti=on  | pti=off |  23100  |      84.3%    |     95.5%    |
>  pti=on  | pti=on  |  13300  |      48.5%    |     55.0%    |

New run :

    Host  |  Guest  | conn/s  | ratio  | Notes
 ---------+---------+---------+--------+----------------
  pti=off | pti=off |  23100  | 100.0% | VM reference without PTI
  pti=off | pti=on  |  19700  |  85.2% | VM with PTI and PCID
  pti=off | pti=on  |  12700  |  55.0% | VM with PTI without PCID

So the performance cut in half was indeed caused by the lack of PCID
here. With it the impact is much less, though still important.

Willy