Re: [PATCH 0/2] Add a new scheme to support demotion on tiered memory system

[Baolin Wang]

On 12/22/2021 4:54 PM, SeongJae Park wrote:
[snip]

> > My machine contains 64G DRAM + 256G AEP(persistent memory), and you
> > should enable the demotion firstly by:
> > echo "true" > /sys/kernel/mm/numa/demotion_enabled
> >
> > Then I just write a simple test case like below to mmap some anon
> > memory, and then just read and write half of the mmap buffer to let
> > another half to be cold enough to demote.
> >
> > int main()
> > {
> >           int len = 50 * 1024 * 1024;
> >           int scan_len = len / 2;
> >           int i, ret, j;
> >           unsigned long *p;
> >
> >           p = mmap(NULL, len, PROT_READ | PROT_WRITE,
> >                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> >           if (p == MAP_FAILED) {
> >                   printf("failed to get memory\n");
> >                   return -1;
> >           }
> >
> >           for (i = 0; i < len / sizeof(*p); i++)
> >                   p[i] = 0x55aa;
> >
> >           /* Let another half of buffer to be cold */
> >           do {
> >                   for (i = 0; i < scan_len / sizeof(*p); i++)
> >                           p[i] = 0x55aa;
> >
> >                   sleep(2);
> >
> >                   for (i = 0; i < scan_len / sizeof(*p); i++)
> >                           j +=  p[i] >> 2;
> >           } while (1);
> >
> >           munmap(p, len);
> >           return 0;
> > }
> >
> > After setting the atts/schemes/target_ids, then start monitoring:
> > echo 100000 1000000 1000000 10 1000 > /sys/kernel/debug/damon/attrs
> > echo 4096 8192000 0 5 10 2000 5 1000 2097152 5000 0 0 0 0 0 3 2 1 >
> > /sys/kernel/debug/damon/schemes
> >
> > After a while, you can check the demote statictics by below command, and
> > you can find the demote scheme is applied by demoting some cold pages to
> > slow memory (AEP) node.
> >
> > cat /proc/vmstat | grep "demote"
> > pgdemote_direct 6881
>
> Thank you for sharing this great details!
>
> I was just wondering if you have tested and measured the effects of the memory
> allocation latency increase during the page demotion, which invoked by
> shrink_page_list(), and also if you have measured how much improvement can be
> achieved with DAMON-based demotion in the scenario.  Seems that's not the case,

Not yet testing on the real workload with DAMON demote scheme now, and I 
think DAMON is lack of some functions to tune performance on tiered 
memory system. At least I think we also need add a new promotion scheme 
for DAMON to promote hot memory from slow memory node to the fast memory 
node, which is on my TODO list.

> and I personally think that information is not essential for this patch, so I
> see no problem here.  But, if you have tested or have a plan to do that, and if
> you could, I think sharing the results on this cover letter would make this
> even greater.

Sure, will do if we find some funny results with DAMON on tiered memory 
system in future. Thanks.