On Thu, Mar 1, 2018 at 11:07 PM, James Chacon <chacon.ja...@gmail.com> wrote:
> > > On Thu, Mar 1, 2018 at 10:13 AM, Ian Lance Taylor <i...@golang.org> wrote: > >> On Thu, Mar 1, 2018 at 8:56 AM, James Chacon <chacon.ja...@gmail.com> >> wrote: >> > >> > I know the time package includes support for using the cycle timer on >> the >> > machine (if available) to get high precision monotonic time >> measurements. >> > >> > But...calling time.Now() appears to have a lot of overhead. Measuring >> the >> > delay between 2 consecutive calls gives me anywhere from 150ns to 900+ns >> > depending on arch (linux and OS/X for these 2 examples). >> > >> > My problem is I'm writing an emulator for an 8 bit cpu and on certain >> types >> > of emulation I want it to run at original clock speeds (so 550ns clock >> > cycles or so in this case). Just measuring time.Now() at the start of a >> > cycle and then subtracting time.Now() at the end to sleep for remaining >> > won't work if the overhead of the calls exceeds my cycle time like it's >> > doing on OS/X. I'm assuming negligible enough overhead for time.Sleep(). >> > >> > I know for benchmarking we deal with this by aggregating a lot of >> samples >> > and then dividing out. Is there a way to get the timer data much >> quicker? >> > I'm assuming on OS/X it's ending up doing the syscall for gettimeofday >> (I >> > recall an open bug somewhere) which is where the large jump comes from. >> > >> > Or should I just measure my average latency when initializing my >> emulator >> > and use that as a baseline for determining how much to sleep? i.e. >> > effectively a mini benchmark on startup to determine local machine >> average >> > run time and assume some slop? >> >> I don't think there is any way we could make time.Now run noticeably >> faster on Darwin. It's not doing a system call of any sort. >> >> > I'm reading time·now in the 1.9.2 sources and it clearly has a fallback > path to invoking the gettimeofday call. > > I hadn't looked at 1.10 yet so I'll update and check my results there. > > >> Your best bet, if you can assume you are running on amd64, is a tiny >> bit of assembly code to execute the rdtsc instruction. rdtsc has its >> problems, but it will give you fairly accurate cycle time when it's >> not way way off. >> >> > I may go with that. > > Wow...Ok, updating to 1.10 made a *huge* difference here. On 1.9.2 Darwin was showing an average of 811ns for back to back time.Now() calls. On 1.10 it's 11-13ns :) I haven't tested amd64 linux yet but I'll assume it's similar. At that amount I can just go back to calling time.Now() at the start/end and sleeping for the difference modula some slop. James -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
