Re: [RFC PATCH v14] sys_membarrier(): system/process-wide memory barrier (x86)

Mathieu Desnoyers
2015-03-18T16:52:18Z

----- Original Message -----
> On Wed, Mar 18, 2015 at 12:23:02PM -0400, Mathieu Desnoyers wrote:
> > memory barriers in reader: 1701557485 reads, 3129842 writes
> > signal-based scheme: 9825306874 reads, 5386 writes
> > sys_membarrier: 7992076602 reads, 220 writes
> >
> > The dynamic sys_membarrier availability check adds some overhead to
> > the read-side compared to the signal-based scheme, but besides that,
> > with the expedited scheme, we can see that we are close to the read-side
> > performance of the signal-based scheme. However, this non-expedited
> > sys_membarrier implementation has a much slower grace period than signal
> > and memory barrier schemes.
>
> Doesn't the query flag allow you to find out in advance rather than
> dynamically within the reader? What's the reader performance if you
> hardcode availability of membarrier?

What I am currently doing is to use sys_membarrier with a query
flag within a lib constructor, and cache the result in a global
variable. In the reader, I just test the variable, and thus detect
whether I can use sys_membarrier, or if I need to fallback to
barriers on both reader and writer.

Are you suggesting I try removing the global variable load+test
from the reader fast path ?

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.