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Date: Tue, 31 Dec 2013 09:45:32 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Initial hashing function. Feedback welcome

On Tue, Dec 31, 2013 at 09:23:21AM +0400, Solar Designer wrote:
> On Tue, Dec 31, 2013 at 08:23:57AM +0400, Solar Designer wrote:
> > If your "pages" do not correspond to a whole number of physical pages,
> > you'll run into false sharing between your threads when they happen to
> > write into the same page.  This can truly kill performance, and this
> > effect and its worst case extent won't be easily predictable.  Slowdowns
> > by many orders of magnitude are possible.
> > 
> > http://en.wikipedia.org/wiki/False_sharing
> 
> Correction: normally, false sharing refers to accesses to the same cache
> line rather than merely the same page.

This 20 year old paper:

http://research.microsoft.com/apps/pubs/default.aspx?id=69629
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.17.3255&rep=rep1&type=pdf (PDF)

says in the abstract: "False sharing occurs when processors in a
shared-memory parallel system make references to different data objects
within the same coherence block (cache line or page)", so it does
include "page" as an option in the definition.  However, I doubt that
modern CPUs use pages rather than solely cache lines as a "coherence
block", so it could be a slightly different yet related issue.

Here's when and how I first ran into the issue in 2010:

http://openwall.info/wiki/internal/gcc-local-build#Parallel-processing-with-GCC

"Unfortunately, reducing N such that the array would fit in the L1 data
cache prevented automatic parallelization from working.  What could the
reason for this be?  I actually tried forcing the compiler into
parallelizing the loop even for smaller values of N by using OpenMP
directives, and the performance was poor - all 8 threads running, but so
slowly that there was no advantage overall.  Apparently, this slowness
was caused by cache coherence overhead.  I expected this problem to occur
with multiple threads accessing the same cache line.  My test results
demonstrated that it was worse than that: if a thread wrote to a given
page of memory, accesses to that entire page by other threads would be
slow (entry thrown out of TLBs maybe?)  I did not spend time to confirm
(or disprove) this guess via the CPU's performance counters, though.

The problem did not occur for read-only accesses [...]"

For a non-artificial program where this issue occurred as well, read the
"Long story short [...]" paragraph further on the same wiki page.  I ran
into (what felt like) this same issue several times since then, testing
on different machines (and CPU types), with the same workaround working.

Alexander

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