Re: [PATCH 0/2] mm: Enable page parallel initialisation for Power

[Li Zhang]

On Tue, Mar 8, 2016 at 10:45 PM, Balbir Singh <bsingharora@xxxxxxxxx> wrote:
>
>
> On 08/03/16 14:55, Li Zhang wrote:
>> From: Li Zhang <zhlcindy@xxxxxxxxxxxxxxxxxx>
>>
>> Uptream has supported page parallel initialisation for X86 and the
>> boot time is improved greately. Some tests have been done for Power.
>>
>> Here is the result I have done with different memory size.
>>
>> * 4GB memory:
>>     boot time is as the following:
>>     with patch vs without patch: 10.4s vs 24.5s
>>     boot time is improved 57%
>> * 200GB memory:
>>     boot time looks the same with and without patches.
>>     boot time is about 38s
>> * 32TB memory:
>>     boot time looks the same with and without patches
>>     boot time is about 160s.
>>     The boot time is much shorter than X86 with 24TB memory.
>>     From community discussion, it costs about 694s for X86 24T system.
>>
>> From code view, parallel initialisation improve the performance by
>> deferring memory initilisation to kswap with N kthreads, it should
>> improve the performance therotically.
>>
>> From the test result, On X86, performance is improved greatly with huge
>> memory. But on Power platform, it is improved greatly with less than
>> 100GB memory. For huge memory, it is not improved greatly. But it saves
>> the time with several threads at least, as the following information
>> shows(32TB system log):
>>
>> [   22.648169] node 9 initialised, 16607461 pages in 280ms
>> [   22.783772] node 3 initialised, 23937243 pages in 410ms
>> [   22.858877] node 6 initialised, 29179347 pages in 490ms
>> [   22.863252] node 2 initialised, 29179347 pages in 490ms
>> [   22.907545] node 0 initialised, 32049614 pages in 540ms
>> [   22.920891] node 15 initialised, 32212280 pages in 550ms
>> [   22.923236] node 4 initialised, 32306127 pages in 550ms
>> [   22.923384] node 12 initialised, 32314319 pages in 550ms
>> [   22.924754] node 8 initialised, 32314319 pages in 550ms
>> [   22.940780] node 13 initialised, 33353677 pages in 570ms
>> [   22.940796] node 11 initialised, 33353677 pages in 570ms
>> [   22.941700] node 5 initialised, 33353677 pages in 570ms
>> [   22.941721] node 10 initialised, 33353677 pages in 570ms
>> [   22.941876] node 7 initialised, 33353677 pages in 570ms
>> [   22.944946] node 14 initialised, 33353677 pages in 570ms
>> [   22.946063] node 1 initialised, 33345485 pages in 580ms
>>
>> It saves the time about 550*16 ms at least, although it can be ignore to compare
>> the boot time about 160 seconds. What's more, the boot time is much shorter
>> on Power even without patches than x86 for huge memory machine.
>>
>> So this patchset is still necessary to be enabled for Power.
>>
>>
>
Hi Balbir,

Thanks for your reviewing.

> The patchset looks good, two questions
>
> 1. The patchset is still necessary for
>     a. systems with smaller amount of RAM?
       I think it is. Currently, I tested systems for 4GB, 50GB, and
boot time is improved.
       We may test more systems with different memory size in the future.
>     b. Theoretically it improves boot time?
       The boot time is improved a little bit for huge memory system
and it can be ignored.
       But I think it's still necessary to enable this feature.

> 2. the pgdat->node_spanned_pages >> 8 sounds arbitrary
>     On a system with 2TB*16 nodes, it would initialize about 8GB before calling deferred init?
>     Don't we need at-least 32GB + space for other early hash allocations
>     BTW, My expectation was that 32TB would imply 32GB+32GB of large hash allocations early on

      pgdat->node_spanned_pages >> 8 means that it allocates the size
of the memory on one node.
      On a system with 2TB *16nodes, it will allocate 16*8GB = 128GB.
      I am not sure if it can be minimised to >> 16 to make sure all
the architectures with different
      memory size work well.  And this is also mentioned in early
discussion for X86, so I choose  >> 8.

*    From the code as the following:

      free_area_init_core ->
                     memmap_init->
                              update_defer_init
     #define memmap_init(size, nid, zone, start_pfn) \
           memmap_init_zone((size), (nid), (zone), (start_pfn), MEMMAP_EARLY)

     memmap_init_zone is based on a zone, but free_area_init_core will
help find the highest
     zone on the node. And update_defer_init() get max initialised
memory on highest zone for a node to
     reserve for early initialisation.

     static void __paginginit free_area_init_core(struct pglist_data *pgdat)
     {
            ...
           for (j = 0; j < MAX_NR_ZONES; j++) {
                  ....
                 memmap_init(size, nid, j, zone_start_fn);   //find
the highest zone on a node.
                 ...
           }
     }

*   From the dmesg log, after applying this patchset, it has
123013440K(about 117GB),
    which is enough for Dentry node hash table and Inode hash table in
this system.

    [    0.000000] Memory: 123013440K/31739871232K available (8000K
kernel code, 1856K rwdata,
    3384K rodata, 6208K init, 2544K bss, 28531136K reserved, 0K cma-reserved)

Thanks :)

>
> Balbir Singh.


-- 

Best Regards
-Li