On 14 Nov 2019, at 18:22, Ludwig Krispenz <lkrispen(a)redhat.com>
wrote:
Hi William,
before further thinking about this, I need some clarification, or maybe I just missed
this. When you talk about 1..16 threads do you mean worker threads ?
Server worker threads. ldclt is set to only use 10 client threads - which is surprising
that with 10 client threads we see a decline when workers > 10 (one would assume it
should stabilise).
Or concurrent client connection threads in ldclt/rsearch/.... - how
many concurrent connections do you have and how does varying this number change results ?
I will add more tests to this to allow varying the ldclt numbers.
Regards,
Ludwig
On 11/14/2019 03:34 AM, William Brown wrote:
> 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.
>
> <Mail Attachment.png>
>
> —
> Sincerely,
>
> William Brown
>
> Senior Software Engineer, 389 Directory Server
> SUSE Labs
>
>
>
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Sincerely,
William Brown
Senior Software Engineer, 389 Directory Server
SUSE Labs