Re: lkwt in DragonFly

[Miguel Mendez <flynn@xxxxxxxxxxxxxxxxxx>]

Jeroen Ruigrok/asmodai wrote:

> I prefer to go with a hybrid method.  1:1 doesn't work.

Could you elaborate on that? I agree that the prefered implementation is
something KSE/SA alike, but Solaris is moving to 1:1 after years of 
trying to make N:M work properly. The main problem I see with 1:1 
threading is kernel memory usage on heavily threaded applications, but 
other than that? Implementation is a lot simpler than in the N:M case. 
The SA idea looks very good on paper until you start implementing and 
see it's actually pretty hard work to get it functioning properly.

> I've got some papers I've been reading on threads lately which I'll
> put in a follow-up tonight.

I assume you've read the Scheduler Activations, which is the basis for 
NetBSD's threading implementation.

> Basically it shows that pure userland threads have pros and cons,
> pure kernel threads have pros and cons and a hybrid mapping is the
> best of both worlds on a number of areas.

Pure userland is fast, but if a thread blocks on i/o your entire process 
does. That seems to be the main rationale behind N:M designs. Having a 
threading system that doesn't have the contention of user->kernel 
switches for thread scheduling and, on the other side, be able to keep 
going when a thread blocks in a syscall.

Cheers,
--
	Miguel Mendez <flynn@xxxxxxxxxxxxxxxxxx>
	http://www.energyhq.es.eu.org
	PGP Key: 0xDC8514F1