On 7/21/14, 10:20, "Darren Hart" <dvhart@xxxxxxxxxxxxxxx> wrote: >On 7/21/14, 9:45, "Andi Kleen" <andi@xxxxxxxxxxxxxx> wrote: > >>Andi Kleen <andi@xxxxxxxxxxxxxx> writes: >> >>> Waiman Long <Waiman.Long@xxxxxx> writes: >>> >>>> This patch series introduces two new futex command codes to support >>>> a new optimistic spinning futex for implementing an exclusive lock >>>> primitive that should perform better than the same primitive using >>>> the wait-wake futex in cases where the lock owner is actively working >>>> instead of waiting for I/O completion. >>> >>> How would you distinguish those two cases automatically? >> >>Also BTW traditionally the spinning was just done in user space. >> >>This would be always even more efficient, because it would >>even avoid the syscall entry path. >> >>Given the glibc adaptive mutexes are not very good, but I'm sure those >>could be improved. > >I presented on something along these lines a few years back, and various >people have asked for the sample code over the years. Andi is right, >doing >this from user-space has the potential to be more efficient, the >challenge >is getting access to privileged information, such as the state of the >mutex owner. You don't want to spin if the owner is sleeping. I did this >by adding a tidrunning call to the vdso. My somewhat hacked solution is >still available here: > >http://git.infradead.org/users/dvhart/linux.git/shortlog/refs/heads/futex/ >t >idrunning/dev > > >I abandoned the spinning in the kernel thing due to the overhead of the >system call if I remember correctly. Also available here: > >http://git.infradead.org/users/dvhart/linux.git/shortlog/refs/heads/futex/ >f >utex-lock-adaptive > And the associated Linux Plumbers 2010 presentation which I can't seem to find archived anywhere else: http://dvhart.com/darren/files/kernel-assisted-userspace-adaptive-mutexes.p df We observed some significant improvements under some very specific use cases, but a more thorough dive into performance impact in the other cases as well as security implications with the vdso is still wanting. -- Darren Hart Open Source Technology Center darren.hart@xxxxxxxxx Intel Corporation -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
