On Tue, Oct 29, 2013 at 3:57 PM, Alexei Starovoitov <alexei.starovoi...@gmail.com> wrote: > On Tue, Oct 29, 2013 at 3:00 PM, Jesse Gross <je...@nicira.com> wrote: >> On Tue, Oct 29, 2013 at 11:19 AM, Alexei Starovoitov >> <alexei.starovoi...@gmail.com> wrote: >>> On Tue, Oct 29, 2013 at 11:15 AM, Pravin Shelar <pshe...@nicira.com> wrote: >>>> On Tue, Oct 29, 2013 at 10:24 AM, Andy Zhou <az...@nicira.com> wrote: >>>>> This is not a review, but it would be nice to add some code comments in >>>>> proper data structure layout, so that the same logic can be followed in >>>>> the >>>>> future. >>>>> >>>> ok, I will add general guideline in datapath.h as follows: >>>> >>>> /* >>>> * data structure members should be arranged according to frequency of >>>> access >>>> * in ovs fast-path. So that most frequently accessed members are in >>>> first cache-line >>>> * for a datapath object. >>>> */ >>> >>> that may be fine as general rule, but I don't see how it helps to >>> avoid cache misses. >> >> It reduces cache pressure by not polluting cache lines with data that >> won't be commonly accessed. > > exactly. "commonly accessed" is not the same as "sorted by frequency". > if field b accessed right after a, you want them to be in the same > cache line and ordered as a,b > If struct has a lot of fields, sorting them by number of accesses may > actually degrade performance. > Some compilers actually able to do field reordering with profile > feedback and they don't sort fields by frequency. > As a general rule, if 'b' accessed usually after 'a', you want them to > be in the order: a,b > Even if overall 'b' accessed more frequently than 'a', since fill of > the cache line is not instant and comes in chunks, for ordered stores > cpu can do store coalescing.
That's generally true but it's not clear to me what specifically you are proposing to be done. _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
