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

Mathieu Desnoyers
2015-03-18T18:50:43Z

----- Original Message -----
> On Wed, Mar 18, 2015 at 04:52:14PM +0000, Mathieu Desnoyers wrote:
> > ----- 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 ?
>
> Right. You said that "The dynamic sys_membarrier availability check
> adds some overhead to the read-side compared to the signal-based
> scheme"; I wondered how much.

With 8 reader threads in parallel, no writer (workload found
in userspace RCU tests/benchmark/test_urcu*.c):

* memory barriers in read-side

307.4 million reads/s

* sys_membarrier read-side

With dynamic check: 1142.0 million reads/s
Hardcoded barrier(): 1453.2 million reads/s (For a 27% speedup over dynamic check.)

* QSBR (quiescent-state based) read-side

2276.9 million reads/s

It might start being worthwhile to consider turning memory barriers
into no-op within lib constructors at some point. Remember that
rcu_read_lock/unlock can be inlined into applications, which may
add to the challenge.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.