Linux 预取数据以缓存 x86-64
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Prefetching data to cache for x86-64
提问by pythonic
In my application, at one point I need to perform calculations on a large contiguous block of memory data (100s of MBs). What I was thinking was to keep prefetching the part of the block my program will touch in future, so that when I perform calculations on that portion, the data is already in the cache.
在我的应用程序中，有一次我需要对一大块连续的内存数据（数百 MB）执行计算。我的想法是继续预取我的程序将来会接触的那部分块，这样当我对该部分执行计算时，数据已经在缓存中了。
Can someone give me a simple example of how to achieve this with gcc? I read _mm_prefetchsomewhere, but don't know how to properly use it. Also note that I have a multicore system, but each core will be working on a different region of memory in parallel.
有人能给我一个简单的例子来说明如何使用 gcc 实现这一目标吗？我在_mm_prefetch某处阅读，但不知道如何正确使用它。另请注意，我有一个多核系统，但每个核将并行处理不同的内存区域。
采纳答案by Jens Gustedt
gccuses builtin functions as an interface for lowlevel instructions. In particular for your case __builtin_prefetch. But you only should see a measurable difference when using this in cases where the access pattern is not easy to predict automatically.
gcc使用内置函数作为低级指令的接口。特别是对于您的情况__builtin_prefetch。但是，只有在访问模式不容易自动预测的情况下使用它时，您才会看到可测量的差异。
回答by Paul R
Modern CPUs have pretty good automatic prefetch and you may well find that you do more harm than good if you try to initiate software prefetching. There is most likely a lot more "low hanging fruit" that you can focus on for optimisation if you find that you actually have a performance problem. Prefetch tends to be one of the last things that you might try, when you're desperate for a few more percent throughput.
现代 CPU 具有很好的自动预取功能，如果您尝试启动软件预取，您很可能会发现弊大于利。如果您发现确实存在性能问题，则很可能还有更多“悬而未决的果实”可以专注于优化。当您迫切需要更高的吞吐量时，预取往往是您可能尝试的最后一件事。
