Hi Chuck, Have tested with both on a server that is up and running already servicing clients and also right after restart. Don't think thread creation time is the bottleneck here.
I don’t have Linux access currently. If I manage to test, I will let you know. Regards, John -----Original Message----- From: Chuck Caldarale <n82...@gmail.com> Sent: Thursday, August 7, 2025 9:01 PM To: Tomcat Users List <users@tomcat.apache.org> Subject: Re: Tomcat acceptCount limitation and connection refused errors CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe. > On 2025 Aug 7, at 18:16, John Williams > <john.willi...@eginnovations.com.INVALID> wrote: > > I am aware that the TCP backlog queue and concurrent connections are > different. The problem occurs before the connection is fully established. > > My testing scenario is: > - A total of 500 connections are attempted concurrently using 500 threads > from a client application. These connections get a simple page and the > response should be instantaneous. > - About 10-40% of the connections are refused (varies from one test run to > another - but it's never 0). > - Tomcat threads does not reach anywhere close to 500. > - Changing the acceptCount in Tomcat's connector has no effect. > - netstat shows connections reaching less than 50. > - The testing is based on the latest available downloadable version of JDK 21 > and Tomcat 11. > - From what I can see, the bottleneck is NOT in Tomcat. > - I have tested after restarting tomcat and making sure there are no other > requests to the application other than the 500 connections I have initiated. Do you do any warm up of the server before running the actual test? There will be some slowness in getting the necessary server threads created, even if it’s only 50 or thereabouts. For curiosity’s sake, would It be possible to run your test application on Linux with <Connector acceptCount=“200”> to match the Windows default setting? I’m wondering if the problem may simply be the thread scheduling overhead in Windows. - Chuck > The Connector configuration is below: > > <Connector executor="tomcatPool" > protocol="org.apache.coyote.http11.Http11Nio2Protocol" port="443" > > sslImplementationName="org.apache.tomcat.util.net.jsse.JSSEImplementation" > scheme="https" secure="true" SSLEnabled="true" > enableLookups="false" acceptCount="12000" > acceptorThreadCount="8" connectionTimeout="300000" maxPostSize="-1" > maxKeepAliveRequests="1" maxConnections="5000" URIEncoding="UTF-8" > useSendfile="false" tcpNoDelay="true" compression="force" > compressionMinSize="2048" compressibleMimeType=" > text/html,text/xml,text/plain, > application/octet-stream,application/xml,text/javascript,text/css,image/png, > application/x-javascript,application/javascript,application/json,application/xhtml+xml,application/xml+xhtml" > noCompressionUserAgents="gozilla, traviata" > > <SSLHostConfig honorCipherOrder="true" ciphers=",..." > certificateVerification="none" protocols="TLSv1.2+TLSv1.3" > sslProtocol="TLS"><Certificate certificateKeystoreFile="binfile" > certificateKeyAlias="cert" certificateKeystorePassword="..." > /></SSLHostConfig> > </Connector> > > Regards, > > > -----Original Message----- > From: Chuck Caldarale <n82...@gmail.com <mailto:n82...@gmail.com>> > Sent: Thursday, August 7, 2025 6:26 PM > To: Tomcat Users List <users@tomcat.apache.org > <mailto:users@tomcat.apache.org>> > Subject: Re: Tomcat acceptCount limitation and connection refused errors > > CAUTION: This email originated from outside of the organization. Do not click > links or open attachments unless you recognize the sender and know the > content is safe. > > >> On 2025 Aug 6, at 18:29, John Williams >> <john.willi...@eginnovations.com.INVALID> wrote: >> >> I have Tomcat 11 running on Windows. When I get a spurt of connections, the >> client sees "Connection Refused" errors sporadically. I have tried >> increasing the acceptCount for the connector and that does not help. >> >> From what I am reading, this appears to be a because of the default TCP >> Backlog setting at the OS level. Windows has this limit set to 200, so if >> there are more than 200 simultaneous connection attempts, some connections >> get refused. >> >> I have verified that there is no CPU/memory bottleneck on the server, >> ephemeral port usage is very low, so this is definitely a TCP backlog issue. >> >> On Windows, there appears to be a way to increase the TCP backlog queue . >> This setting is at the socket level, rather than an OS setting. >> >> Does Tomcat provide a way to increase the OS level TCP backlog queue length, >> and if not, can this be considered in the roadmap? >> >> This seems to be a scalability issue for Tomcat - 200 concurrent connection >> limit appears to be a very low number. Please correct me if I am wrong. > > > The backlog queue size is not the limit for the number of concurrent > connections - they are distinct entities. A connections request sits in the > backlog queue until the server accepts it, which usually occurs extremely > quickly - unless the actual concurrency limit as specified in your > <Connector> element has been reached. The backlog limit only comes into play > if Tomcat cannot accept additional connections for some reason, such as the > thread limit being reached. > > Since you didn't provide your Tomcat <Connector> configuration, no actual > answers can be provided - just speculation. > > - Chuck > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org > <mailto:users-unsubscr...@tomcat.apache.org> > For additional commands, e-mail: users-h...@tomcat.apache.org > <mailto:users-h...@tomcat.apache.org> > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org > <mailto:users-unsubscr...@tomcat.apache.org> > For additional commands, e-mail: users-h...@tomcat.apache.org > <mailto:users-h...@tomcat.apache.org>
