> On Mar 31, 2021, at 6:16 AM, Mel Gorman <mgorman@xxxxxxx> wrote: > > On Wed, Mar 31, 2021 at 07:20:09PM +0800, Huang, Ying wrote: >> Mel Gorman <mgorman@xxxxxxx> writes: >> >>> On Mon, Mar 29, 2021 at 02:26:51PM +0800, Huang Ying wrote: >>>> For NUMA balancing, in hint page fault handler, the faulting page will >>>> be migrated to the accessing node if necessary. During the migration, >>>> TLB will be shot down on all CPUs that the process has run on >>>> recently. Because in the hint page fault handler, the PTE will be >>>> made accessible before the migration is tried. The overhead of TLB >>>> shooting down is high, so it's better to be avoided if possible. In >>>> fact, if we delay mapping the page in PTE until migration, that can be >>>> avoided. This is what this patch doing. >>>> >>> >>> Why would the overhead be high? It was previously inaccessibly so it's >>> only parallel accesses making forward progress that trigger the need >>> for a flush. >> >> Sorry, I don't understand this. Although the page is inaccessible, the >> threads may access other pages, so TLB flushing is still necessary. >> > > You assert the overhead of TLB shootdown is high and yes, it can be > very high but you also said "the benchmark score has no visible changes" > indicating the TLB shootdown cost is not a major problem for the workload. > It does not mean we should ignore it though. If you are looking for a benchmark that is negatively affected by NUMA balancing, then IIRC Parsec’s dedup is such a workload. [1] [1] https://parsec.cs.princeton.edu/
Attachment:
signature.asc
Description: Message signed with OpenPGP
