[lttng-dev] Practical recommendation for max update rate with QSBR

Thu Jul 21 23:14:18 UTC 2016

Thanks!

      From: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
 To: Mark E. Dawson <medawsonjr at yahoo.com> 
Cc: lttng-dev <lttng-dev at lists.lttng.org>
 Sent: Thursday, July 21, 2016 4:58 PM
 Subject: Re: [lttng-dev] Practical recommendation for max update rate with QSBR

----- On Jul 21, 2016, at 4:47 PM, Mark E. Dawson <medawsonjr at yahoo.com> wrote:

All,
I've read the documentation regarding reader throughput drop-offs with high update rates due to the pointer exchanging between readers-writers, and the general admonition of using URCU only for mostly-read workloads with relatively infrequent updates.
However, is there a general rule-of-thumb suggestion for highest recommended update rate sustainable for optimal performance with URCU (in particular, the QSBR flavor) for highly threaded applications deployed on high core count machines (Intel)? The example case would be a single updater and 20 - 30 reader threads.

Hi Mark,

We have some relevant measurements in the supplementary material of:

Desnoyers, Mathieu, McKenney, Paul. E., Stern, Alan S., Dagenais, Michel R. and Walpole, Jonathan, User-Level Implementations of Read-Copy Update. IEEE Transaction on Parallel and Distributed Systems, 23 (2): 375-382 (2012).
A copy can be found at http://www.efficios.com/publications

See p. 10 of supplementary material,  Fig. 14:

"Comparison of Pointer Exchange and Ideal RCU Update Overhead, 8-core Intel Xeon, Logarithmic Scale"

Since you are doing pointer exchange, you will want to look at the "QSBR"line, which stays at 1000M reads/s up to 100000 updates/s, and then startsdropping.
On the 64-core POWER5+ (Fig. 15), read speed starts dropping near 100000updates/s too.

Thanks,
Mathieu

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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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