On 10/9/19 10:00 PM, William Brown wrote:
On 9 Oct 2019, at 19:55, Ludwig Krispenz <lkrispen@xxxxxxxxxx> wrote:
Hi William,
I like your radical approach :-)
In my opinion our connection code is getting to complicated by maintaining two different implementations in parallel - not separated, but intermangled (and even more complicated by turbo mode). So I agree we should have only one, but which one ? In my opinion nunc stans is the theoretically better approach, but nobody is confident enough to rely on nunc stans alone. The conntable mode has its problems (especially if handling many concurrent connections, and worse if they are established almost at the same time)(otherwise we would not have experimented with nunc stans), but is stable and for most of the use cases efficient enough.
I think you nailed it in one - we aren't confident in nunc-stans today, so let's keep what works and improve that. There are already many similar concepts - work queues, threads, even slapi_conn_t. I think that it would be possible to bring "nunc-stans ideas" into reworking and improvement to the current connection code instead.
So reducing the complexity by removing nunc stans (and maybe also turbo mode) and then do cleanup and try to improve the bottlenecks would be an acceptable approach to me.
Agree. It also means we can make much smaller changes in an easier to control and test fashion I think.
In my opinion the core of the problem of the "old" connection code is that the main thread is handling new connections and already established connections and so does iterate over the connection table. Using an event model looks like the best way to handle this, but if it doesn't work we need to look for other improvements without breaking things.
Your suggestion to make the conn table data structure more lean and flexible is one option. In sun ds, when I didn't know about event queues I did split the main thread, one handling new connections and multiple to handle established connections (parts of teh conn table) - reusing the existing mechanisms, just splitting the load. Maybe we can also think in this direction.
I think so too. We can certainly have some ideas about what actually does the polling vs what does accepting, or better, event management etc. There are some ideas to have smaller groups of workers too to improve thread locality and help improve concurrency too.
So maybe I'll put together a patch to remove nunc-stans soon then, and start to look at the existing connection code and options to improve that + some profiling.
I still would like to hear about my original question though as quoted below, I think Mark might have some comments :)
Why nunc-stans? Because at the time we were terrible at handling many
connections, simultaneously and in large numbers. C10K, etc. Our
competitors performed much better in these scenarios than we did. I
recall several customer cases complaining about "our performance verses
theirs" in regards to large numbers of connection.
So, moving forward, I agree with everyone here. We should remove
nunc-stans, but as William said try to incorporate some of its concepts
into our connection code (eventually). We should clean up the
connection code as much as possible, and remove turbo mode if it does
not provide much value. The one thing that has been frustrating is how
the connection code had become very complicated and most of us no longer
know how it works anymore. It would be nice to get it to a state that
is much more maintainable (especially for engineers new to the code).
I think we should look at improving the connection table as previously
suggested, and we should add the additional polling thread that Ludwig
mentioned. We know we will get added performance with just these two
changes. Then we should stress and evaluate the new behavior, and if
need be we can look into more invasive/architectural changes and use
some of the ideology from nunc-stans. (This also means we need a way to
properly and consistently test high connection load from multiple clients).
Mark
The main question is *why* do we want it merged?
Is it performance? Recently I provided a patch that yielded an approximate ~30% speed up in the entire server through put just by changing our existing connection code.
Is it features? What features are we wanting from this? We have no complaints about our current threading model and thread allocations.
Is it maximum number of connections? We can always change the conntable to a better datastructure that would help scale this number higher (which would also yield a performance gain).
—
Sincerely,
William Brown
Senior Software Engineer, 389 Directory Server
SUSE Labs
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