On Sat, Jun 09, 2018 at 06:07:15PM -0700, Don Lewis wrote: > On 9 Jun, Stefan Esser wrote: > > > 3) Programs that evenly split the load on all available cores have been > > suffering from sub-optimal assignment of threads to cores. E.g. on a > > CPU with 8 (virtual) cores, this resulted in 6 cores running the load > > in nominal time, 1 core taking twice as long because 2 threads were > > scheduled to run on it, while 1 core was mostly idle. Even if the > > load was initially evenly distributed, a woken up process that ran on > > one core destroyed the symmetry and it was not recovered. (This was a > > problem e.g. for parallel programs using MPI or the like.) > > When a core is about to go idle or first enters the idle state it will > search for the most heavily loaded core and steal a thread from it. The > core will only go to sleep if it can't find a non-running thread to > steal. > > If there are N cores and N+1 runnable threads, there is a long term load > balancer than runs periodically. It searches for the most and least > loaded cores and moves a thread from the former to the latter. That > prevents the same pair of threads from having to share the same core > indefinitely. > > There is an observed bug where a low priority thread can get pinned to a > particular core that is already occupied by a high-priority CPU-bound > thread that never releases the CPU. The low priority thread can't > migrate to another core that subsequently becomes available because it > it is pinned. It is not known how the thread originally got into this > state. I don't see any reason for 4BSD to be immune to this problem. >
It is a well-known problem that an over-subscribed ULE kernel has much worse performance than a 4BSD kernel. I've posted more than once with benchmark numbers that demonstrate the problem. -- Steve _______________________________________________ freebsd-current@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
