RFR: 8294610: java/net/vthread/HttpALot.java is slow on Linux

[Alan Bateman]

On Fri, 30 Sep 2022 09:10:05 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

> Can I please get a review for this test-only change which proposes to improve the test run duration of `java/net/vthread/HttpALot.java` test, on Linux? This relates to https://bugs.openjdk.org/browse/JDK-8294610
> 
> Experiments have shown that on Linux, due to delayed TCP ACKs from the client (for some previous sent data/packet), the server sent response waits for the ACKs before sending the response data (in a TCP packet). Each delayed ACK is 40 milli seconds and given the number of requests this test (intentionally) issues, the delay accumulates causing the test to take longer duration.
> 
> With the change in this PR, the test completes in around 6 to 8 seconds on a Linux system as compared to 60 odd seconds previously on the same setup. This change hasn't shown any negative impact on macos (which continues to run this test in around 5 to 6 seconds like previously). I will trigger some runs in our CI setup to make sure this works as expected in all other OS too.
> 
> As noted in the linked JBS issue, additional experiments to see if anything can be improved in the JDK network layer will be carried out separately, outside  the context of this test.

test/jdk/java/net/vthread/HttpALot.java line 32:

> 30:  * @library /test/lib
> 31:  * @compile --enable-preview -source ${jdk.version} HttpALot.java
> 32:  * @comment The test launch command intentionally uses -Dsun.net.httpserver.nodelay=true

Maybe replace this "The test runs with -Dsun.net.httpserver.nodelay=true" as the words "launch command intentionally uses" aren't really needed.

test/jdk/java/net/vthread/HttpALot.java line 36:

> 34:  *          to avoid (occasional 40 milli seconds) delays in receiving responses from the server.
> 35:  *          Given the number of requests being fired in this test, the delayed responses can
> 36:  *          accumulate into longer test run duration

It might be clearer to say that "to avoid occasional 40ms delays receiving responses from the server on Linux.". I think you can drop "Given the number ..." sentence.

test/jdk/java/net/vthread/HttpALot.java line 39:

> 37:  * @run main/othervm/timeout=600
> 38:  *     --enable-preview
> 39:  *     -Dsun.net.httpserver.nodelay=true

good, I'm happy this issue was found and the HTTP server already has a way to change this option.

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PR: https://git.openjdk.org/jdk/pull/10504