Hi all,
After our catch up, we were discussing performance matters. I decided to start
on this while waiting for some of my tickets to be reviewed and to see what's
going on.
These tests were carried out on a virtual machine configured in search 6 to
have access to 6 CPU's, and search 12 with 12 CPU. Both machines had access to
8GB of ram.
The hardware is an i7 2.2GHz with 6 cores (12 threads) and 32GB of ram, with
NVME storage provided.
The rows are the VM CPU's available, and the columns are the number of threads
in nsslapd-threadnumber. No other variables were changed. The database has 6000
users and 4000 groups. The instance was restarted before each test. The search
was a randomised uid equality test with a single result. I provided the thread
6 and 12 columns to try to match the VM and host specs rather than just the
traditional base 2 sequence we see.
I've attached a screen shot of the results, but I have some initial thoughts to
provide on this. What's interesting is our initial 1 thread performance and how
steeply it ramps up towards 4 thread. This in mind it's not a linear increase.
Per thread on s6 we go from ~3800 to ~2500 ops per second, and a similar ratio
exists in s12. What is stark is that after t4 we immediately see a per thread
*decline* despite the greater amount of available computer resources. This
indicates that it is poor locking and thread coordination causing a rapid
decline in performance. This was true on both s6 and s12. The decline
intesifies rapidly once we exceed the CPU avail on the host (s6 between t6 to
t12), but still declines even when we do have the hardware threads available in
s12.
I will perform some testing between t1 and t6 versions to see if I can isolate
which functions are having a growth in time consumption.
For now an early recommendation is that we alter our default CPU auto-tuning.
Currently we use a curve which starts at 16 threads from 1 to 4 cores, and then
tapering down to 512 cores to 512 threads - however in almost all of these
autotuned threads we have threads greater than our core count. This from this
graph would indicate that this decision only hurts our performance rather than
improving it. I suggest we change our thread autotuning to be 1 to 1 ratio of
threads to cores to prevent over contention on lock resources.
Thanks, more to come once I setup this profiling on a real machine so I can
generate flamegraphs.
—
Sincerely,
William Brown
Senior Software Engineer, 389 Directory Server
SUSE Labs
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