RFR: 8294610: java/net/vthread/HttpALot.java is slow on Linux [v2]

[Jaikiran Pai]

> 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.

Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:

  update test comment to address review comments

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10504/files
  - new: https://git.openjdk.org/jdk/pull/10504/files/ed3878d2..76c870f5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10504&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10504&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10504.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10504/head:pull/10504

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