Messages in this thread | | | From | Jesse Gross <> | Date | Thu, 28 Apr 2011 18:48:32 -0700 | Subject | Re: [PATCH] flex_array: Avoid divisions when accessing elements. |
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On Wed, Apr 27, 2011 at 9:13 AM, Dave Hansen <dave@linux.vnet.ibm.com> wrote: > On Tue, 2011-04-26 at 13:41 -0700, Jesse Gross wrote: >> On most architectures division is an expensive operation and >> accessing an element currently requires four of them. This >> performance penalty effectively precludes flex arrays from >> being used on any kind of fast path. However, two of these >> divisions can be handled at creation time and the others can >> be replaced by a reciprocal divide, completely avoiding real >> divisions on access. > > flex_array.c has a nice table for how many objects can be allocated: > > * Element size | Objects | Objects | > * PAGE_SIZE=4k | 32-bit | 64-bit | > * ---------------------------------| > * 1 bytes | 4186112 | 2093056 | > * 2 bytes | 2093056 | 1046528 | > * 3 bytes | 1395030 | 697515 | > * 4 bytes | 1046528 | 523264 | > * 32 bytes | 130816 | 65408 | > * 33 bytes | 126728 | 63364 | > * 2048 bytes | 2044 | 1022 | > * 2049 bytes | 1022 | 511 | > * void * | 1046528 | 261632 | > > This patch changes that a bit. Would you mind updating it?
Sure, I'll send out an updated patch that does that tomorrow. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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