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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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.