On Tue, Jan 6, 2026 at 5:59 AM Morten Brørup <[email protected]> wrote: > > > From: Scott Mitchell <[email protected]> > > > > Optimize __rte_raw_cksum() by processing data in larger unrolled loops > > instead of iterating word-by-word. The new implementation processes > > 64-byte blocks (32 x uint16_t) in the hot path, followed by smaller > > 32/16/8/4/2-byte chunks. > > Good idea processing in 64-byte blocks! > > I wonder if there would be further gain by 64-byte aligning the 64-byte > chunks, so the compiler can use vector instructions for summing the 32 2-byte > words of each 64-byte chunk. > This would require a 3-step algorithm: > 1. Process the first 0..63 bytes preceding the first 64-byte aligned address. > (These bytes are unaligned; nothing new here.) > 2. Process 64-byte chunks, if any. These are now 64-byte aligned, and you > should ensure that the compiler knows it. > 3. Process the last 32/16/8/4/2/1-byte chunks. These are now aligned, which > eliminates the need for unaligned_uint16_t in this step. Specifically, the > 32-byte chunk will be 64-byte aligned, allowing the compiler to use vector > instructions. The 16-byte chunk will be 32-byte aligned. Etc. > > <random idea> > Step 1 may be performed in reverse order of step 3, i.e. process in chunks of > 1/2/4/8/16/32 bytes (using the lowest bits of the address as condition) - > which will cause the alignment to increase accordingly. > </random idea> > > <feature creep> > Checking the alignment at runtime has a non-zero cost, so a an alternative > (simpler) code path might be beneficial for small lengths (when the alignment > is unknown at runtime). > </feature creep> >
Good idea! I implemented your suggestion but I didn't observe a measurable difference in cksum_perf_autotest. I suggest we proceed with the approach in this patch as an incremental step and I can post a followup with your suggestion above to review/discuss. Note the checksum computation requires processing in 16 bit blocks for correctness which requires special case handling for odd length/buffer-address alignment so complexity/code is higher. > > > > Uses uint64_t accumulator to reduce carry propagation overhead > > You return (uint32_t)sum64 at the end, so why replace the existing 32-bit > "sum" with a 64-bit "sum64" accumulator? Good catch. It gives more headroom to avoid overflow but not necessary and I will revert. > > > and > > leverages unaligned_uint16_t for safe unaligned access on all > > platforms. > > > > Performance results from cksum_perf_autotest (TSC cycles/byte): > > Block size Before After Improvement > > 100 0.40-0.64 0.13-0.14 ~3-4x > > 1500 0.49-0.51 0.10-0.11 ~4-5x > > 9000 0.48-0.51 0.11-0.12 ~4x > > > > Signed-off-by: Scott Mitchell <[email protected]> >
