Messages in this thread | | | Date | Tue, 31 Jan 2012 09:57:26 -0800 | Subject | Re: [PATCH] mtd/nand: use string library | From | Brian Norris <> |
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On Fri, Jan 27, 2012 at 3:52 PM, Akinobu Mita <akinobu.mita@gmail.com> wrote: > 2012/1/28 Brian Norris <computersforpeace@gmail.com>: >> On Fri, Jan 27, 2012 at 9:16 AM, Joe Perches <joe@perches.com> wrote: >>> On Fri, 2012-01-27 at 23:24 +0900, Akinobu Mita wrote: >>>> - Use memchr_inv to check if the data contains all 0xFF bytes. >>>> It is faster than looping for each byte. >>> >>> Stupid question: >>> >>> Are there any mtd devices modified that are slower >>> at 64 bit accesses than repeated 8 bit accesses? >> >> I believe this patch deals with kernel buffers, not any kind of direct >> access to the MTD, so the question (which is not stupid IMO) should be >> regarding CPU architectures. And my educated guess is that 64-bit >> access should not be any slower. I do know that 8-bit access *is* >> slower for some relevant architectures. > > It could be slower when the number of bytes scanned is very small > (found a unmatched character immediately, or the size of the area > is very small), because memchr_inv() needs to generate a 64bit pattern > to compare before starting the loop. I recalled that Eric Dumazet > pointed out it could generate the 64bit pattern more efficiently. > (https://lkml.org/lkml/2011/8/8/480) > > Even if that small scanning is slower, this change can be assumed cleanup > patch that simplifies the code.
Well, I agree that it qualifies as cleanup as well, but we should at least make an attempt not to cause performance regression...
So by my understanding, the use of memchr_inv() is on buffers of minimum length of 10 in this patch, so we're likely to have decent results. And memcmp() usage looks fine to me.
So unless other concerns arise:
Acked-by: Brian Norris <computersforpeace@gmail.com> -- 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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