SMP & HTT on 6.3 (P4)

Jeremie Le Hen jeremie at
Thu Feb 14 02:07:02 PST 2008


On Tue, Feb 05, 2008 at 01:08:10AM +0200, Todorov wrote:
>  Hash: SHA1
>  Hi,
>  what do you think for HyperThreading (P4 GHz), which serves FBSD 6.3?
>  Now it's disabled by the BIOS but since today I've upgraded the machine
>  5.5 to 6.3 though if under 6.XX series it worths or not.
>  I've read for performance penalties under 5.XX series w/ HTT on. I'm not
>  going to change to ULE (it's said to be better for HTT awareness).

ULE is broken on RELENG_6.  Forget it.  If you really want ULE, you'll
have to upgrade to RELENG_7.

Regarding HTT, I haven't heard of any performance improvement with HTT
on FreeBSD.  The scheduler have to be aware of it because HTT processors
share the same cache lines (including L1).  Unenlightened scheduling
leads to an increase of cache miss (42% according [1]).  This is not the
case on any FreeBSD branch currently.  Nonetheless Jeff Roberson is
working on a CPU topology-aware scheduler implementation [2] but this is
not even in -CURRENT yet.  It will probably ends up in RELENG_8.

Besides, Colin Percival has shown that HTT could lead to local
information leak [3].

WRT to these informations, I advice you to disable HTT.


Jeremie Le Hen
< jeremie at le-hen dot org >< ttz at chchile dot org >

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