Messages in this thread | | | Date | Thu, 16 Feb 2006 14:23:48 -0600 | From | "Christopher Friesen" <> | Subject | Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace? |
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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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