On Mon, Sep 10, 2018 at 01:30:11PM -0700, Dave Hansen wrote: >On 09/08/2018 06:38 PM, owner-linux-mm@xxxxxxxxx wrote: >> >> At last, here is the test result on my 4G virtual machine. I added printk >> before and after sparse_memory_present_with_active_regions() and tested three >> times with/without "likely". >> >> without with >> Elapsed 0.000252 0.000250 -0.8% >> >> The benefit seems to be too small on a 4G virtual machine or even this is not >> stable. Not sure we can see some visible effect on a 32G machine. > >I think it's highly unlikely you have found something significant here. >It's one system, in a VM and it's not being measured using a mechanism >that is suitable for benchmarking (the kernel dmesg timestamps). > >Plus, if this is a really tight loop, the cpu's branch predictors will >be good at it. Hi, Dave Thanks for your reply. I think you are right. This part is not significant and cpu may do its job well. Hmm... I am still willing to hear your opinion on my analysis of this situation. In which case we would use likely/unlikely. For example, in this case the possibility is (255/ 256) if the system has 32G RAM. Do we have a threshold of the possibility to use likely/unlikely. Or we'd prefer not to use this any more? Let compiler and cpu do their job. Look forward your insights. -- Wei Yang Help you, Help me
