Re: revivesa status 2008/07/09

[Mindaugas Rasiukevicius <rmind%NetBSD.org@localhost>]

Thor Lancelot Simon <tls%rek.tjls.com@localhost> wrote:
> I think there's another significant benefit: it should give a significant,
> possibly even huge, performance benefit for multithreaded applications on
> platforms which are uniprocessor and have a significant penalty for
> context-switching into the kernel.

Would not be so sure about significant performance benefit. Can you provide
a practical application where SA outperforms 1:1?

Theoretically, I see some quite specific cases where SA may perform good,
but doubt that in real, practical world such cases have a use, or if have,
I believe there are other ways to achieve the same without SA (as we are
talking about workload based in the user-space).

Note that multithreaded applications usually need synchronisation, which
likely involves blocking, otherwise - I am not sure why engineer have chosen
the multithreaded way in specific UP system for CPU-only workload.

> The obvious example is ARM -- and there are a _lot_ of ARM CPUs out
> there in the world in embedded devices, often running software that makes
> heavy use of threading for its UI.  There are pretty well-understood reasons
> why SA actually does win in this kind of use case, and it's not an
> unimportant one for NetBSD.

Well, it is "well-understood" in theory. SA was a kind of nice idea from
theoretical perspective, but was not proven practically in any relevant
environment and system. At least two systems which tried migrated to 1:1.

Also, you have mentioned "threading for its UI", what usually means a lot of
blocking (and/or synchronisation), so it is not a workload for SA.

> If I saw a benchmark -- and not an I/O bound one -- from such a platform
> where the 1:1 threading did anywhere near as well as the SA threading I'd
> certainly change my mind, but...

Perhaps you can give us a link to that benchmark which would show how SA
performs better than 1:1? There were many benchmarks which proven 1:1
performance, but unfortunately I have not seen any benchmark where SA
performs better than 1:1 on NetBSD.

It is easy to say "well-understood", "obvious", "saw". But perhaps we can
be more constructive by providing numbers, test-applications, or something
like that?

-- 
Best regards,
Mindaugas
www.NetBSD.org