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Subject: [Boost-users] IEEE blurb on multi-core performance, comments regarding "threads" and speed.
From: Mike Marchywka (marchywka_at_[hidden])
Date: 2008-11-15 08:35:19
This is quite close to being a political rant
but I thought it was worth the risk of annoying the moderator :)
Many people ask about threads, presumably trageting single or maybe 2-4 core processors,
expecting a speed up proportional to the number of threads. I keep
posting questions about boost support for cache or at least memory awareness
and the needs for locality ( IIRC, it was hard to keep an early 2 core
hyperthread chip fed without a little help from vtune ).
The graph here is quite dramatic ( but I think their units "seconds" needs a minus one if
you believe the text, in any case the red curve is NOT monotonic ):
"At the heart of the trouble is the so-called memory wallthe growing disparity between how fast a CPU can operate on data and how fast it can get the data it needs. Although the number of cores per processor is increasing, the number of connections from the chip to the rest of the computer is not. So keeping all the cores fed with data is a problem. In informatics applications, the problem is worse, explains Richard C. Murphy, a senior member of the technical staff at Sandia, because there is no physical relationship between what a processor may be working on and where the next set of data it needs may reside. Instead of being in the cache of the core next door, the data may be on a DRAM chip in a rack 20 meters away and need to leave the chip, pass through one or more routers and optical fibers, and find its way onto the processor. "
So, as you design apps or libary code, think globally and act locally. Don't assume that an ignorant
but hopefully "fair" thread scheduler will save you from cache thrashing that should be fixed by
more attention to algorithm design.
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