On 3.04.19 г. 11:54 ч., Qu Wenruo wrote:
> Hi,
>
> Recently Intel LKP performance test is reporting regression of btrfs
> performance.
>
> It points to tree-checker code, and since I'm poking around the
> bcc/ebpf, I spend some time to do an interesting look into the
> performance penalty about both btrfs csum and tree-checker.
>
> The code base is David's misc-next, which contains both write-time tree
> checker and enhanced code to handle fuzzed image.
>
> The tool can be find in my gist:
> https://gist.github.com/adam900710/b5542f2e52ed4687986cf41f64b85253
So you are essentially trying to figure out the average run time of 3
functions, this could have been made simpler by using the funclatency
bcc tool from iovisor repo:
https://github.com/iovisor/bcc/blob/master/tools/funclatency.py
Actually running this tool will show you a latency histogram making it
easier to spot any latency outliers. An average value doesn't mean
anything without having more context i.e stddev.
>
> To use the tool, one needs bcc-python binding and kernel config for
> eBPF, but at least Arch default kernel has all needed config, so any one
> can try it on Arch.
>
> The work load is:
> mkfs.btrfs -n 4K $DEV
> mount $DEV $MNT
> fsstress -n 10000 -w -d $MNT
> umount $MNT
>
> ## start my script ##
> mount $DEV $MNT
> ls -R $MNT > /dev/null # To read all fs tree blocks
> fsstress -n 1000 -w -d $MNT # Trigger enough write
> umount $MNT
> ## stop my script ##
>
>
> The result is very interesting:
> Basic result is:
> CSUM_TREE_BLOCK: nr=2311 total=10000612 avg=4327
> TREE_CHECKER_READ: nr=461 total=41911553 avg=90914
> TREE_CHECKER_WRITE: nr=1575 total=5783330 avg=3671
Definitely something worth looking at.
>
> So if just looking at the average number of csum calculate, it only
> brings 3~5μs. And surprisingly, write time tree checker even slower than
> checksum!
>
> Also surprisingly, read time tree checker takes near 100μs. nearly 20
> times slower than csum/write time tree checker.
>
> So we have a new direction to enhance tree-checker performance.
> BTW, bcc/ebpf is really awesome!
>
> Thanks,
> Qu
>
