* Mel Gorman <[email protected]> wrote:
> > So have you explored the possibility to significantly simplify your
> > patch-set
> > by only deferring the flushing, and doing a simple TLB flush on the remote
> > CPU?
>
> Yes. At one point I looked at range flushing but it is not a good idea.
My suggestion wasn't range-flushing, but a simple all-or-nothing batched flush
of
user-space TLBs.
> The ranges that reach the end of the LRU are too large to be useful except in
> the ideal case of a workload that sequentially accesses memory. Flushing the
> full TLB has an unpredictable cost. [...]
Why would it have unpredictable cost? We flush the TLB on every process context
switch. Yes, it's somewhat workload dependent, but the performance profile is
so
different anyway with batching that it has to be re-measured anyway.
> With a full flush we clear entries we know were recently accessed and may
> have
> to be looked up again and we do this every 32 mapped pages that are
> reclaimed.
> In the ideal case of a sequential mapped reader it would not matter as the
> entries are not needed so we would not see the cost at all. Other workloads
> will
> have to do a refill that was not necessary before this series. The cost of
> the
> refill will depend on the CPU and whether the lookup information is still in
> the
> CPU cache or not. That means measuring the full impact of your proposal is
> impossible as it depends heavily on the workload, the timing of its
> interaction
> with kswapd in particular, the state of the CPU cache and the cost of refills
> for the CPU.
>
> I agree with you in that it would be a simplier series and the actual flush
> would probably be faster but the downsides are too unpredictable for a series
> that primarily is about reducing the number of IPIs.
Sorry, I don't buy this, at all.
Please measure this, the code would become a lot simpler, as I'm not convinced
that we need pfn (or struct page) or even range based flushing.
I.e. please first implement the simplest remote batching variant, then
complicate
it if the numbers warrant it. Not the other way around. It's not like the VM
code
needs the extra complexity!
Thanks,
Ingo
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