On Jun 26, 2014, at 10:55 AM, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > On Thu, 2014-06-26 at 16:53 +0200, Bart Van Assche wrote: >> On 06/11/14 11:09, Sagi Grimberg wrote: >>> + return xfer_len + (xfer_len >> ilog2(sector_size)) * 8; >> >> Sorry that I just noticed this now, but why is a shift-right and ilog2() >> used in the above expression instead of just dividing the transfer >> length by the sector size ? > > It's a performance thing. Division is really slow on most CPUs. > However, we know the divisor is a power of two so we re-express the > division as a shift, which the processor can do really fast. > > James I have done this in the past as well, but have you benchmarked it? Compilers typically do the right thing in this case (i.e replace division with shift). Scott-- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
