On 3/18/2022 6:49 PM, sj@xxxxxxxxxx wrote:
On Fri, 18 Mar 2022 18:01:19 +0800 Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
On 3/18/2022 5:40 PM, sj@xxxxxxxxxx wrote:
Hi Baolin,
On Fri, 18 Mar 2022 17:23:13 +0800 Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
When I try to sample the physical address with DAMON to migrate pages
on tiered memory system, I found it will demote some cold regions mistakenly.
Now we will choose an physical address in the region randomly, but if
its corresponding page is not an online LRU page, we will ignore the
accessing status in this cycle of sampling, and actually will be treated
as a non-accessed region. Suppose a region including some non-LRU pages,
it will be treated as a cold region with a high probability, and may be
merged with adjacent cold regions, but there are some pages may be
accessed we missed.
So instead of ignoring the access status of this region if we did not find
a valid page according to current sampling address, we can use last valid
sampling address to help to make the sampling more accurate, then we can do
a better decision.
Well... Offlined pages are also a valid part of the memory region, so treating
those as not accessed and making the memory region containing the offlined
pages looks colder seems legal to me. IOW, this approach could make memory
regions containing many non-online-LRU pages as hot.
IMO I don't think this is a problem, since if this region containing
many non-online-LRU pages is treated as hot, which means threre are aome
pages are hot, right? We can find them and promote them to fast memory
(or do other schemes). Meanwhile, for non-online-LRU pages, we can
filter them and do nothing for them, since we can not get a valid page
struct for them.
For some of DAMOS actions that you mentioned, that could make sense. However,
that wouldn't make much sense for some other cases, especially for manual
DAMON-based access pattern profiling.
I am not sure about this case, could you elaborate on how this can worse
the case you mentioned?
Like you said as below, we can split the regions to separate the hot
pages out of the hot regions containing some offline or non-lru pages,
that is also a benefit to improve the regions adjustment.
After all, we already have a mechanism for this case: adaptive regions
adjustment (or, regions split/merge). That mechanism will eventually separate
out hot oneline-LRU pages in the memory regions. Before the region is
adjusted, reporting the whole region as hot looks like a right result to me.
Of course, I admit that it could take too much time to converge to the optimal
regions, and there are many rooms for improvement of the regions adjustment
mechanism. I think we should pursue the direction (improving the regions
adjustment mechanism).
Yes, agree.
FYI, I have some rough ideas for improving the mechanism including partitioning
regions into more than 2 sub-regions if we belive it is not making a good
progress. Nevertheless, I'd like to first make a methodology for evaluating
current accuracy. For that, I am planning to implement a page-granularity
access monitoring.
Great, I think the page-granularity monitoring will be more suitable for
tiered memory system, which can reduce redundant demotion and promotion.
However, I still concern the overhead if the monitoring is a
page-granularity, especially for a large memory size. Anyway, I'd like
to help to test or review the new page-granularity monitoring when
you're ready to send out. Thanks.
