Re: [PATCH 13/40] autonuma: CPU follow memory algorithm

[Rik van Riel <riel@redhat.com>]

On 07/02/2012 03:56 AM, Nai Xia wrote:
>
>
> On 2012a1'07ae??02ae?JPY 15:36, Rik van Riel wrote:
>> On 06/30/2012 11:10 AM, Nai Xia wrote:
>>
>>> Yes, pte_numa or pte_young works the same way and they both can
>>> answer the problem of "which pages were accessed since last scan".
>>> For LRU, it's OK, it's quite enough. But for numa balancing it's NOT.
>>
>> Getting LRU right may be much more important than getting
>> NUMA balancing right.
>>
>> Retrieving wrongly evicted data from disk can be a million
>> of times slower than fetching data from RAM, while the
>> penalty for accessing a remote NUMA node is only 20% or so.
>>
>>> We also should care about the hotness of the page sets, since if the
>>> workloads are complex we should NOT be expecting that "if this page
>>> is accessed once, then it's always in my CPU cache during the whole
>>> last scan interval".
>>>
>>> The difference between LRU and the problem you are trying to deal
>>> with looks so obvious to me, I am so worried that you are still
>>> messing them up :(
>>
>> For autonuma, it may be fine to have a lower likelyhood of
>> obtaining an optimum result, because the penalty for getting
>> it wrong is so much lower.
>
> I said, I am actually want to see some detailed analysis
> showing that this sampling is really playing an important role
> in benchmarks as it claims to be. Not a quick
> "lower likelyhood than optimum" conclusion.....
>
> Please, Rik, I know your points, you don't have to explain
> anymore. But I just cannot follow without research data.

What kind of data are you looking for?

I have seen a lot of generic comments in your emails,
and one gut feeling about Andrea's sampling algorithm,
but I seem to have missed the details of exactly what
you are looking for.

Btw, I share your feeling that Andrea's sampling
algorithm will probably not be able to distinguish
between NUMA nodes that are very frequent users of
a page, and NUMA nodes that use the same page much
less frequently.

However, I suspect that the penalty of getting it
wrong will be fairly low, while the overhead of
getting access frequency information will be
prohibitively high. There is a reason nobody uses
LRU nowadays, but a clock style algorithm instead.


-- 
All rights reversed

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>