6:31 p.m.
On Thu, Jul 26, 2018 at 5:27 AM R. Matt Barnett <barnett(a)rice.edu> wrote:
I've been able to observe 1...8 on Red Hat by adding the
following
statements to my handler (and setting the worker thread pool size to 8):
@Overridepublic void handleRequest(HttpServerExchange httpServerExchange) throws
Exception
{
if (httpServerExchange.isInIoThread()) {
httpServerExchange.dispatch(this);
return;
}
...
}
I have a few questions about this technique though:
1.) How are dispatch actions mapped onto worker threads? New connections were not mapped
to available idle IO threads, so is it possible dispatches also won't be mapped to
available idle worker threads but instead queued for currently
busy threads?
IO threads are tied to the connection. Once a connection has been accepted
only that IO thread will be used to service it. This avoids contention from
having a larger number of IO threads waiting on a single selector. The
worker thread pool is basically just a normal executor, that will run tasks
in a FIFO manner.
2.) The Undertow documentation states that HttpServerExchange is not thread-safe. However
the documentation states that dispatch(...) has happens-before semantics with respect to
the worker thread accessing httpServerExchange.
That would seem to make it ok to read from httpServerExchange in the worker thread.
Assuming that an IO thread will be responsible for writing the http response back to the
client, what steps do I need to take in the body
of handleRequest to ensure that my writes to httpServerExchange in the worker thread are
observed by the IO thread responsible for transmitting the response to the client? Is
invoking httpServerExchange.endExchange(); in the
worker thread as the final statement sufficient?
Not all writes are done from the IO thread. For instance if you use
blocking IO and are using a Stream then the writes are done from the worker.
If you use the Sender to perform async IO then the initial write is done
from the original thread, and the IO thread is only involved if the
response is too larger to just write out immediately. In this case though
the Sender will take care of the thread safety aspects, as the underlying
SelectionKey will not have its interest ops set until after the current
stack has returned.
Basically if you call dispatch(), or perform an action that requires async
IO nothing happens immediately, it just sets a flag in the
HttpServerExchange. Once the call stack returns (i.e. the current thread is
done) one of three things will happen:
- If dispatch was called the dispatch task will be run in an executor
- If async IO was required the underlying SelectionKey will have its
interest ops modified, so the IO thread can perform the IO
- If neither of the above happened then the exchange is ended.
Stuart
-- Matt
On 7/25/2018 11:26 AM, R. Matt Barnett wrote:
Corrected test to resolve test/set race.
https://gist.github.com/rmbarnett-rice/1179c4ad1d3344bb247c8b8daed3e4fa
I've also discovered this morning that I *can* see 1-8 printed on Red
Hat when I generate load using ab from Windows, but only 1-4 when
running ab on Red Hat (both locally and from a remote server). I'm
wondering if perhaps there is some sort of connection reuse shenanigans
going on. My assumption of the use of the -c 8 parameter was "make 8
sockets" but maybe not. I'll dig in and report back.
-- Matt
On 7/24/2018 6:56 PM, R. Matt Barnett wrote:
Hello,
I'm experiencing an Undertow performance issue I fail to understand. I
am able to reproduce the issue with the code linked bellow. The problem
is that on Red Hat (and not Windows) I'm unable to concurrently process
more than 4 overlapping requests even with 8 configured IO Threads.
For example, if I run the following program (1 file, 55 lines):
https://gist.github.com/rmbarnett-rice/668db6b4e9f8f8da7093a3659b6ae2b5
... on Red Hat and then send requests to the server using Apache
Benchmark...
> ab -n 1000 -c 8 localhost:8080/
I see the following output from the Undertow process:
Server started on port 8080
1
2
3
4
I believe this demonstrates that only 4 requests are ever processed in
parallel. I would expect 8. In fact, when I run the same experiment on
Windows I see the expected output of
Server started on port 8080
1
2
3
4
5
6
7
8
Any thoughts as to what might explain this behavior?
Best,
Matt
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