Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace?

[Christopher Friesen]

Esben Nielsen wrote:
> On Thu, 16 Feb 2006, Arjan van de Ven wrote:
>
>
> > On Thu, 2006-02-16 at 20:06 +0100, Esben Nielsen wrote:

> > > Why does the list have to be in userspace?
> >
> > because it's faster ;)

> Faster??? 
> As I see it, extra manipulations have to be done even in the non-congested
> case: Every time the lock is taken the locking thread has to add the lock
> to a the list, and reversely remove the lock from the list. I.e.
> instructions are _added_ to the fast path where you stay purely in
> userspace.
>
> I am ofcourse comparing to a solution where you do a syscall on everytime
> you do a lock.


The whole *point* of futexes is that on uncontested operations you don't 
have to do a syscall.  Thus, if you can avoid taking a syscall while 
still getting reliability, you'll be faster.

Dropping to kernelspace isn't free.

Chris

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