On Thu, Oct 11, 2012 at 12:01:37PM +0100, Mel Gorman wrote: > On Thu, Oct 04, 2012 at 01:50:46AM +0200, Andrea Arcangeli wrote: > > The objective of _PAGE_NUMA is to be able to trigger NUMA hinting page > > faults to identify the per NUMA node working set of the thread at > > runtime. > > > > Arming the NUMA hinting page fault mechanism works similarly to > > setting up a mprotect(PROT_NONE) virtual range: the present bit is > > cleared at the same time that _PAGE_NUMA is set, so when the fault > > triggers we can identify it as a NUMA hinting page fault. > > > > That implies that there is an atomic update requirement or at least > an ordering requirement -- present bit must be cleared before setting > NUMA bit. No doubt it'll be clear later in the series how this is > accomplished. What you propose seems ok but it all depends how it's > implemented so I'm leaving my ack off this particular patch for now. Correct. The switch is done atomically (clear _PAGE_PRESENT at the same time _PAGE_NUMA is set). The tlb flush is deferred (it's batched to avoid firing an IPI for every pte/pmd_numa we establish). It's still similar to setting a range PROT_NONE (except the way _PAGE_PROTNONE and _PAGE_NUMA works is the opposite, and they are mutually exclusive, so they can easily share the same pte/pmd bitflag). Except PROT_NONE must be synchronous, _PAGE_NUMA is set lazily. The NUMA hinting page fault also won't require any TLB flush ever. So the whole process (establish/teardown) has an incredibly low TLB flushing cost. The only fixed cost is in knuma_scand and the enter/exit kernel for every not-shared page every 10 sec (or whatever you set the duration of a knuma_scand pass in sysfs). Furthermore, if the pmd_scan mode is activated, I guarantee there's at max 1 NUMA hinting page fault every 2m virtual region (even if some accuracy is lost). You can try to set scan_pmd = 0 in sysfs and also to disable THP (echo never >enabled) to measure the exact cost per 4k page. It's hardly measurable here. With THP the fault is also 1 every 2m virtual region but no accuracy is lost in that case (or more precisely, there's no way to get more accuracy than that as we deal with a pmd). -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>