Messages in this thread | | | Date | Tue, 15 Sep 2015 08:11:02 +0200 | From | Ingo Molnar <> | Subject | Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal due to a kernel NULL pointer dereference) |
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* Christoph Lameter <cl@linux.com> 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.
Unfortunately, GCC overdoes -Os generating outright silly code, which makes the result generally slower - despite the reduced instruction count and reduced cache footprint.
We've recently applied patches to the x86 tree that give us a good chunk of the size savings that -Os brings:
52648e83c9a6 x86: Pack loops tightly as well be6cb02779ca x86: Align jump targets to 1-byte boundaries
these two shave about 5% off from the typical distro kernel's size. That's still way off the 15%-20% that -Os can muster, but another ~10% are possible by not aligning functions to byte boundaries (instead of the default 16 bytes).
So about 70% of the -Os size win is from simple and pure alignment relaxation, not from any deeper compiler optimizations.
So LLVM could emulate most of the good effects of -Os by only compressing the various alignment parameters - and this would be a pretty safe optimization as well.
Thanks,
Ingo
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