Date: Tue, 25 Apr 2006 09:48:21 -0400 From: Chuck Swiger <cswiger@mac.com> To: Bill Moran <wmoran@collaborativefusion.com> Cc: freebsd-questions@freebsd.org, Derek Ragona <derek@computinginnovations.com> Subject: Re: Purchasing the correct hardware: dual-core intel? Big cache? Message-ID: <444E28A5.3010902@mac.com> In-Reply-To: <20060425092526.6fe5efa6.wmoran@collaborativefusion.com> References: <20060424154617.9dc28c94.wmoran@potentialtech.com> <6.0.0.22.2.20060424175443.02927f48@mail.computinginnovations.com> <20060425084752.2453c0f1.wmoran@collaborativefusion.com> <6.0.0.22.2.20060425075227.028aea10@mail.computinginnovations.com> <20060425092526.6fe5efa6.wmoran@collaborativefusion.com>
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Bill Moran wrote: [ ... ] >> If you use well optimized applications, you see the larger performance >> gain. Poor optimization causes a CPU to chug along, flushing the CPU cache >> often, and slowing things down considerably. > > I know. That's why I'm so desperately trying to find a way to determine > how often the cache is being invalidated - so I can determine whether > larger cache sizes (such as 8M) are worthwhile. Guys, you're confusing two things: "flushing the pipeline" vs. "L2 cache hit ratio". The former happens when branch prediction/speculative execution goes awry and requires the CPU to clear the pipeline of partially-executed instructions and backtrack to follow the other path. It is related to optimization quality of compilers, but is not related at all to how big your L2 cache is. The size of your L2 cache affects how much data is more local to the CPU than main memory, and increasing it will improve the L2 cache hit ratio, or, equivalently, reduce L2 cache misses. This is affected by some specific compiler optimizations (cf "loop unrolling"), but tends to reflect the specifics of the workload and how much multitasking of different programs you do more than the compiler. -- -Chuck
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