If implemented properly adcx/adox should give additional speedup... that is the whole reason for their existence.
Neil Horman <nhor...@tuxdriver.com> wrote: >On Sat, Oct 12, 2013 at 03:29:24PM -0700, H. Peter Anvin wrote: >> On 10/11/2013 09:51 AM, Neil Horman wrote: >> > Sébastien Dugué reported to me that devices implementing ipoib >(which don't have >> > checksum offload hardware were spending a significant amount of >time computing >> > checksums. We found that by splitting the checksum computation >into two >> > separate streams, each skipping successive elements of the buffer >being summed, >> > we could parallelize the checksum operation accros multiple alus. >Since neither >> > chain is dependent on the result of the other, we get a speedup in >execution (on >> > hardware that has multiple alu's available, which is almost >ubiquitous on x86), >> > and only a negligible decrease on hardware that has only a single >alu (an extra >> > addition is introduced). Since addition in commutative, the result >is the same, >> > only faster >> >> On hardware that implement ADCX/ADOX then you should also be able to >> have additional streams interleaved since those instructions allow >for >> dual carry chains. >> >> -hpa >> >I've been looking into this a bit more, and I'm a bit confused. >According to >this: >http://www.intel.com/content/www/us/en/intelligent-systems/intel-technology/ia-large-integer-arithmetic-paper.html > >by my read, this pair of instructions simply supports 2 carry bit >chains, >allowing for two parallel execution paths through the cpu that won't >block on >one another. Its exactly the same as whats being done with the >universally >available addcq instruction, so theres no real speedup (that I can >see). Since >we'd either have to use the alternatives macro to support adcx/adox >here or the >old instruction set, it seems not overly worth the effort to support >the >extension. > >Or am I missing something? > >Neil -- Sent from my mobile phone. Please pardon brevity and lack of formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
