RE: [PATCH] less tlb flush in unmap_vmas

[Chen, Kenneth W]

Nick Piggin wrote on Tuesday, March 21, 2006 8:53 PM
> Shaohua Li wrote:
> >In unmaping region, if current task doesn't need reschedule, don't do a
> >tlb_finish_mmu. This can reduce some tlb flushes.
> >
> >In the lmbench tests, this patch gives 2.1% improvement on exec proc
> >item and 4.2% on sh proc item.
> 
> The problem with this is that by the time we _do_ determine that a
> reschedule is needed, we might have built up a huge amount of work
> to do (which can probably be as much if not more exensive per-page
> as the unmapping), so scheduling latency can still be unacceptable
> so I'm afraid I don't think we can include this patch.

Interesting. In the old day, since mm->page_table_lock is held for the
entire unmap_vmas function, it was beneficial to introduce periodic
reschedule point and to drop the spin lock under pressure. Now that the
page table lock is fine-grained and is pushed into zap_pte_range(), I
would think scheduling latency would improve from lock contention
avoidance point of view.  It is not the case?

- Ken

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