Messages in this thread | | | From | Eric Dumazet <> | Date | Mon, 13 Dec 2021 07:56:20 -0800 | Subject | Re: [PATCH] x86/lib: Remove the special case for odd-aligned buffers in csum_partial.c |
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On Mon, Dec 13, 2021 at 7:37 AM David Laight <David.Laight@aculab.com> wrote: > > From: Dave Hansen > > Sent: 13 December 2021 15:02 > .c > > > > On 12/13/21 6:43 AM, David Laight wrote: > > > There is no need to special case the very unusual odd-aligned buffers. > > > They are no worse than 4n+2 aligned buffers. > > > > > > Signed-off-by: David Laight <david.laight@aculab.com> > > > --- > > > > > > On an i7-7700 misaligned buffers add 2 or 3 clocks (in 115) to a 512 byte > > > checksum. > > > That is just measuring the main loop with an lfence prior to rdpmc to > > > read PERF_COUNT_HW_CPU_CYCLES. > > > > I'm a bit confused by this changelog. > > > > Are you saying that the patch causes a (small) performance regression? > > > > Are you also saying that the optimization here is not worth it because > > it saves 15 lines of code? Or that the misalignment checks themselves > > add 2 or 3 cycles, and this is an *optimization*? > > I'm saying that it can't be worth optimising for a misaligned > buffer because the cost of the buffer being misaligned is so small. > So the test for a misaligned buffer are going to cost more than > and plausible gain. > > Not only that the buffer will never be odd aligned at all. > > The code is left in from a previous version that did do aligned > word reads - so had to do extra for odd alignment. > > Note that code is doing misaligned reads for the more likely 4n+2 > aligned ethernet receive buffers. > I doubt that even a test for that would be worthwhile even if you > were checksumming full sized ethernet packets. > > So the change is deleting code that is never actually executed > from the hot path. >
I think I left this code because I got confused with odd/even case, but this is handled by upper functions like csum_block_add()
What matters is not if the start of a frag is odd/even, but what offset it is in the overall ' frame', if a frame is split into multiple areas (scatter/gather)
Reviewed-by: Eric Dumazet <edumazet@google.com>
Thanks !
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