Messages in this thread | | | Date | Wed, 18 Mar 2015 18:50:41 +0000 (UTC) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH v14] sys_membarrier(): system/process-wide memory barrier (x86) |
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----- 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. http://www.efficios.com
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