Dienstag, 23. Januar 2018

Impact of KPTI on the IO-500 for DKRZ

In a previous post, a first test with the IO-500 has been made on tmpfs to see the impact on the resulting numbers. That experiment had some limitations as it was run on two different nodes.
This time, the experiment is repeated on the same node (btc2 on the DKRZ testystem) and using the exact same kernel but in one case we had KPTI enabled and in the other it was disabled using the debug interface: /sys/kernel/debug/x86/pti_enabled.
Also the overall benchmark suite was not only run 50 times but more than 500 times in each configuration.

Again, 10 processes have been used.


An overview is given in the following table:
Experiment Relative speed with KPTI
ior_easy_write 0.995
mdtest_easy_write 1.011
ior_hard_write 0.991
mdtest_hard_write 1
find 0.973
ior_easy_read 1.002
mdtest_easy_stat 1.003
ior_hard_read 1.015
mdtest_hard_stat 0.999
mdtest_easy_delete 1.001
mdtest_hard_read 0.969
mdtest_hard_delete 0.993

So that means that overall a few experiments now run by 3% slower (find, mdtest_hard_read), but easy write is 1% faster. 

The following graphs provide boxplots of the indiviual repeated measurements with enabled/disabled KPTI:
Fig1: IOR measurements

Fig2: Metadata measurements
While there are some outliers in both configurations, the overal picture looks comparable.


The impact of KPTI is neglectable on our sytem for I/O benchmarks as particularly Lustre is significantly slower than using tmpfs, the results for IO-500 are similar with the results when analyzing latency more fine grained as in my previous post about the latency of individual operations.

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