Messages in this thread | | | Date | Mon, 07 Aug 2006 10:52:10 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86_64: Make NR_IRQS configurable in Kconfig |
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Eric W. Biederman wrote: > "H. Peter Anvin" <hpa@zytor.com> writes: > >> Eric W. Biederman wrote: >>> a) Because I would like to flush out bugs. >>> b) Because I want a default that works for everyone. >>> c) Because with MSI we have a potential for large irq counts on most systems. >>> d) Because anyone who disagrees with me can send a patch and fix >>> the default. >>> e) Because with the default number of cpus we can very close to needing >>> this many irqs in the worst case. >>> f) This is much better than previous to my patch and setting NR_CPUS=255 >>> and getting 8K IRQS. >>> g) Because I probably should have been more inventive than copying the >>> NR_IRQS text, but when I did the wording sounded ok to me. >>> >> Why not simply reserve 224*NR_CPUS IRQs? If you have 256 CPUs allocating 64K >> IRQs should hardly matter :) > > Well there is this little matter of 224*NR_CPUS*NR_CPUS counters at that point > that I think would be prohibitive for most sane people. Taking 224K of per cpu > memory in 256 different per cpu areas. > > Still what is 56MB when you have a terrabyte of RAM. :) >
However, 99.99% of all systems have 16 or fewer CPU cores. Your solution with its proposed default eats more memory for any system with fewer than 19 CPUs.
Furthermore, you don't need 224*NR_CPUS*NR_CPUS counters. If an IRQ is only mapped into one CPU's space it can only be taken on that CPU, thus you only need 224*NR_CPUS counters.
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