Re: Test report for kernel direct mapping performance

Xing Zhengjun
2021-01-27T08:05:02Z

On 1/26/2021 11:00 PM, Michal Hocko wrote:
> On Fri 15-01-21 15:23:07, Xing Zhengjun wrote:
>> Hi,
>>
>> There is currently a bit of a debate about the kernel direct map. Does using
>> 2M/1G pages aggressively for the kernel direct map help performance? Or, is
>> it an old optimization which is not as helpful on modern CPUs as it was in
>> the old days? What is the penalty of a kernel feature that heavily demotes
>> this mapping from larger to smaller pages? We did a set of runs with 1G and
>> 2M pages enabled /disabled and saw the changes.
>>
>> [Conclusions]
>>
>> Assuming that this was a good representative set of workloads and that the
>> data are good, for server usage, we conclude that the existing aggressive
>> use of 1G mappings is a good choice since it represents the best in a
>> plurality of the workloads. However, in a *majority* of cases, another
>> mapping size (2M or 4k) potentially offers a performance improvement. This
>> leads us to conclude that although 1G mappings are a good default choice,
>> there is no compelling evidence that it must be the only choice, or that
>> folks deriving benefits (like hardening) from smaller mapping sizes should
>> avoid the smaller mapping sizes.
>
> Thanks for conducting these tests! This is definitely useful and quite
> honestly I would have expected a much more noticeable differences.
> Please note that I am not really deep into benchmarking but one thing
> that popped in my mind was whethere these (micro)benchmarks are really
> representative workloads. Some of them tend to be rather narrow in
> executed code paths or data structures used AFAIU. Is it possible they
> simply didn't generate sufficient TLB pressure?
>

The test was done on 4 server platforms with 11 benchmarks which 0day
run daily. For the 11 different benchmarks that were used, echo
benchmarks have a lot of subcases, so there was a total of 259 test
cases. The test memory size for the 4 server platform ranges from 128GB
to 512GB. Yes, some of the benchmarks tend to be narrow in executed code
paths or data structures. So we run a total of 259 cases which include
test cases in memory, CPU scheduling, network, io, and database, try to
cover most of the code path. For the 11 benchmarks, some of them may not
generate sufficient TLB pressure, but I think cases in vm-scalability
and will-it-scale may generate sufficient TLB pressure. I have provided
the test results for different benchmarks, if you are interested, you
can see in the details of the test report:
https://01.org/sites/default/files/documentation/test_report_for_kernel_direct_mapping_performance_0.pdf

> Have you tried to look closer on profiles of respective configurations
> where the overhead comes from?
>

The test cases selected from the 0day daily run cases, just use the
different kernel settings;
Enable both 2M and 1G huge pages (up to 1G, so named to "1G" in the test
report):
no extra kernel command line need
Disable 1G pages (up to 2M, so named to 2M in the test report):
add kernel command line "nogbpages"
Disable both 2M and 1G huge pages (up to 4k, so named to 4K in the test
report):
add kernel command line "nohugepages_mapping" (by debug patch)

User spaces add THP enabled setting for all the three kernels (1G/2M/4K)
transparent_hugepage:
thp_enabled: always
thp_defrag: always

During the test, we enabled some monitors, but the overhead should be
not too big, most of the overhead should be the test cases themselves.
I will study some test cases to find the hotspot from which overhead
comes from and provide it later if someone is interested in it.

--
Zhengjun Xing