Ivan Andika created HDDS-16121:
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Summary: Use virtual threads for S3G
Key: HDDS-16121
URL: https://issues.apache.org/jira/browse/HDDS-16121
Project: Apache Ozone
Issue Type: Improvement
Reporter: Ivan Andika
Assignee: Ivan Andika
Currently, S3G use the blocking thread-per-request threading model. This has
the advantage of being simple to implement (as opposed to the asynchronous
event loop model).
However, we encountered performance issues where S3G HTTP threads are blocked
which causes the threads to hit hadoop.http.max.threads and therefore causes
the subsequent requests to be waiting in the waitingTaskCount which increases
the latenncy. This issue can happen even if the downstream services are fine
(i.e. Ozone cluster is not slow or stuck).
For example, if an HTTP client stalled while downloading a key from S3G (e.g.
client machine is slow, etc), the S3G thread will be blocked serving this read
request until hadoop.http.idle_timeout.ms which triggers the "Idle timeout
expired". Check the Jetty IdleTimeout.java for the logic for idle, the idea is
if no data is written in the socket for a while (e.g. notIdle is not invoked),
a scheduled task will fail this request to prevent request hogging the thread
for too long.
{code:java}
SEVERE: An I/O error has occurred while writing a response message entity to
the container output stream.
org.glassfish.jersey.server.internal.process.MappableException:
java.io.IOException: java.util.concurrent.TimeoutException: Idle timeout
expired: 60000/60000 ms
at
org.glassfish.jersey.server.internal.MappableExceptionWrapperInterceptor.aroundWriteTo(MappableExceptionWrapperInterceptor.java:67)
at
org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:139)
at
org.glassfish.jersey.message.internal.MessageBodyFactory.writeTo(MessageBodyFactory.java:1116)
at
org.glassfish.jersey.server.ServerRuntime$Responder.writeResponse(ServerRuntime.java:649)
at
org.glassfish.jersey.server.ServerRuntime$Responder.processResponse(ServerRuntime.java:380)
at
org.glassfish.jersey.server.ServerRuntime$Responder.process(ServerRuntime.java:370)
{code}
Meaning that a single thread can block a single thread although there are still
a lot of CPU capacity left.
One idea is to migrate the Jetty threading model to Servlet 3.1 Asynchronous IO
which implements WriteListener and ReadListener, this would prevent a single
request holding the thread for too long. However, this requires changes in the
S3G implementation which is tedious and risky.
A better solution is to use virtual threads. Since virtual threads are not real
threads, a single OS thread can be mapped to multiple virtual threads and
blocked virtual threads should not cause OS thread to be blocked since OS
thread will pick another virtual thread which can do useful work. Additionally,
since the threading model of virtual thread is similar to OS thread, we don't
need to change threading model so the risk is lower. Of course, there are some
risks in virtual threads (i.e. thread pinning, overloading the downstream
services, etc), so we should still evaluate it or we can make a flag to whether
enable or disable virtual threads and do canary testing.
This requires Jetty upgrade, but Jetty upgrade is overdo (we are still using a
very old 9.x Jetty version) and we should do it asap.
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