Official support for Unsafe

[Maurizio Cimadamore]

On 15/01/2024 15:44, Quân Anh Mai wrote:
> Running the same program on 1e6 lines results in only 9e9 
> instructions, so I think the vast majority of the instruction count is 
> of the compiled code. Not using the universe segment is roughly 
> equivalent to my previous version, which would result in around 50% 
> more instructions compared to using one, and almost double the 
> instruction count of using Unsafe.

Without looking at the program some more, it's hard for me to make some 
sense of these numbers. I'm surprised that you don't see any difference 
when using unbounded segment compared to regular ones. I wonder if the 
gap you are seeing is due to the JVM warming up, rather than peak 
performances being worse. Have you tried measuring peak performance with 
e.g. JMH? I would not expect to see 20% difference there...

Maurizio

>
> Regards,
> Quan Anh
>
> On Mon, 15 Jan 2024 at 23:09, Maurizio Cimadamore 
> <maurizio.cimadamore at oracle.com> wrote:
>
>     I think the increased instruction count is normal, as C2 had to do
>     more work to optimize the bound checks away?
>
>     Is there any difference compared to the version that doesn't use
>     the universe segment?
>
>     Maurizio
>
>     On 15/01/2024 13:52, Quân Anh Mai wrote:
>>     Hi,
>>
>>     I have tried using a universe segment instead of Unsafe, and
>>     store the custom hashmap buffer in off-heap instead of using a
>>     byte array. The output of perf stat on the program
>>
>>      Performance counter stats for 'sh
>>     calculate_average_merykittyunsafe.sh':
>>
>>               13573.70 msec task-clock:u              # 10.942 CPUs
>>     utilized
>>                      0      context-switches:u        #  0.000 /sec
>>                      0      cpu-migrations:u          #  0.000 /sec
>>                 238460      page-faults:u             # 17.568 K/sec
>>            61995179870      cycles:u                  #  4.567 GHz
>>              261830581      stalled-cycles-frontend:u #  0.42%
>>     frontend cycles idle
>>               93823680      stalled-cycles-backend:u  #  0.15%
>>     backend cycles idle
>>           137976098809      instructions:u            #  2.23  insn
>>     per cycle
>>                                                       #  0.00
>>      stalled cycles per insn
>>            18373313803      branches:u                #  1.354 G/sec
>>               43579782      branch-misses:u           #  0.24% of all
>>     branches
>>
>>            1.240504612 seconds time elapsed
>>
>>           12.841563000 seconds user
>>            0.652428000 seconds sys
>>
>>     For comparison, this is the unsafe version:
>>
>>      Performance counter stats for 'sh
>>     calculate_average_merykittyunsafe.sh':
>>
>>               13327.46 msec task-clock:u              #   11.202 CPUs
>>     utilized
>>                      0      context-switches:u        #    0.000 /sec
>>                      0      cpu-migrations:u          #    0.000 /sec
>>                 269896      page-faults:u             #   20.251 K/sec
>>            61258348752      cycles:u                  #    4.596 GHz
>>              639839262      stalled-cycles-frontend:u #    1.04%
>>     frontend cycles idle
>>              108018676      stalled-cycles-backend:u  #    0.18%
>>     backend cycles idle
>>           113476168983      instructions:u            #    1.85  insn
>>     per cycle
>>                                                       #    0.01
>>      stalled cycles per insn
>>            11442665370      branches:u                #  858.578 M/sec
>>               44590172      branch-misses:u           #    0.39% of
>>     all branches
>>
>>            1.189768677 seconds time elapsed
>>
>>           12.628512000 seconds user
>>            0.620083000 seconds sys
>>
>>     This program running on my machine expresses dependency bound so
>>     the difference in execution time is not as significant as on the
>>     test machine but it can be seen that removing Unsafe results in
>>     over 21% increase in instruction count.
>>
>>     Regards,
>>     Quan Anh
>>
>>     On Sat, 13 Jan 2024 at 01:29, Maurizio Cimadamore
>>     <maurizio.cimadamore at oracle.com> wrote:
>>
>>
>>         On 12/01/2024 17:26, Quân Anh Mai wrote:
>>         > FYI, in my submission to 1brc, using Unsafe decreases the
>>         execution
>>         > time from 3.25s to 2.57s on the test machine.
>>
>>         Just curious - what is the difference compared with the
>>         everything
>>         segment trick?
>>
>>         (While I know it can't do on-heap access, perhaps you can
>>         tweak the code
>>         to be all off-heap?)
>>
>>         Maurizio
>>
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