Messages in this thread | | | Subject | Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal due to a kernel NULL pointer dereference) | From | Austin S Hemmelgarn <> | Date | Mon, 14 Sep 2015 14:38:00 -0400 |
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On 2015-09-14 14:27, Christoph Lameter wrote: > On Mon, 14 Sep 2015, Austin S Hemmelgarn wrote: > >> I can comment at least a little about the -Os aspect (although not I'm no >> expert on this in particular). In general, for _most_ use cases, a >> kernel >> compiled with CONFIG_CC_OPTIMIZE_FOR_SIZE will run slower than one >> compiled >> without it. On rare occasion though, it may actually run faster, the >> only >> cases I've seen where this happens are specialized uses that are very >> memory >> pressure dependent and run almost entirely in userspace with almost no >> syscalls (for example math related stuff operating on _very, very big_ >> (as in, >> >1 trillion elements) multidimensional matrices, with complex memory >> constraints), and even then it's usually a miniscule improvement in >> performance (generally less than 1%, which can of course be significant >> depending on how long it takes before the improvement). > > Cache footprint depends on size which has a significant impact on > performance. In our experience the kernel (and any other code) is > generally faster if optimized for size. > Ah, yes, there is that too (like I tried to say, and messed up my grammar in doing so, I'm no expert), although on processors that actually have a reasonable amount of cache, this is not usually something most people would notice without a benchmark except on a very slow processor (HPC workloads and gamers notwithstanding of course).
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