Mark -- Got it. So TCP retransmits can impact tomcat processing time under certain conditions, more likely due to issues with receiving requests from the client than sending responses.
-Eric > -----Original Message----- > From: Mark Thomas <ma...@apache.org> > Sent: Tuesday, September 8, 2020 4:05 PM > To: users@tomcat.apache.org > Subject: Re: Tomcat Processing Timer Question > > On 08/09/2020 21:46, Eric Robinson wrote: > > Hi Mark -- > > > > "If the request is split across multiple packets the timer starts when > > Tomcat > reads the first byte of the request from the first packet. > > Tomcat stops the timer on a request after the last byte of the response has > been accepted by the network stack." > > > > Now we're getting somewhere. If tomcat starts its timer when it reads the > first byte of the client's request, and the request is split into multiple > packets, > doesn't it stand to reason that the timer would run longer when there are > TCP retransmits? > > For the request, it depends. If the retransmit is for part of the request body > and Tomcat hasn't read that far yet (or starting reading at > all) then it probably won't impact the processing time. If Tomcat is > performing a read and waiting for that packet then it will. > > For the response, not unless the response is sfficiently big and the > retransmit > sufficiently earlier in the response that the TCP buffers fill and Tomcat is > blocked from further writes. > > Mark > > > > > > --Eric > > > >> -----Original Message----- > >> From: Mark Thomas <ma...@apache.org> > >> Sent: Tuesday, September 8, 2020 3:34 PM > >> To: users@tomcat.apache.org > >> Subject: Re: Tomcat Processing Timer Question > >> > >> On 08/09/2020 21:19, Eric Robinson wrote: > >>> Hi Mark and Christopher, > >>> > >>> For clarification, suppose a client sends and HTTP POST request > >>> which > >> is bigger than the PMTU and has to be broken into multiple packets. > >> It sounds like you're saying that the request is buffered by the > >> network stack, and the stack does not send it up to tomcat until the full > request is received. > >> That would make sense if every HTTP request is encapsulated in its > >> own separate TCP connection. Most of the time, that is not the case. > >> A single connection is held open and used for multiple HTTP requests. > >> The network stack has no understanding of anything above TCP, so it > >> does not know when an HTTP request complete. It must therefore > >> deliver whatever it has, and it would be up to tomcat to decide when > >> the HTTP request is complete, wouldn't it? > >>> > >>> If that is the case, tomcat could receive a partial HTTP request and > >> would have to wait for the rest before processing it. So when does > >> tomcat start its processing timer? > >> > >> Tomcat starts the processing timer as soon as Tomcat processes the > >> first bytes of the request. In practice, this means the network stack > >> has to deliver the data to Tomcat, the Poller fires a read event, a > >> thread is allocated to process that read event, any TLS handshake has > >> completed and Tomcat has read the first real byte of the request. > >> > >> If the request is split across multiple packets the timer starts when > >> Tomcat reads the first byte of the request from the first packet. > >> > >> Tomcat stops the timer on a request after the last byte of the > >> response has been accepted by the network stack. > >> > >> HTH, > >> > >> Mark > >> > >>> > >>> > >>>> -----Original Message----- > >>>> From: Christopher Schultz <ch...@christopherschultz.net> > >>>> Sent: Tuesday, September 8, 2020 1:19 PM > >>>> To: users@tomcat.apache.org > >>>> Subject: Re: Tomcat Processing Timer Question > >>>> > >>> Eric, > >>> > >>> On 9/8/20 13:46, Eric Robinson wrote: > >>>>>> It is my understanding that the AccessLogValve %D field records > >>>>>> the time from when the last byte of the client's request is > >>>>>> received to when the last byte of the server's response is placed on > the wire. > >>>>>> Is that correct? If so, would TCP retransmissions impact the timer? > >>> > >>> I'm not positive, but I believe Tomcat has zero visibility into that > >>> level of detail. > >>> > >>>>>> If there are connectivity issues between the client and server, > >>>>>> resulting in TCP retransmits, could that appear as higher > >>>>>> response times in the localhost_access logs? > >>> > >>> This would only happen if the re-transmissions were to cause network > >>> buffering in the OS such that the stream writes (at the Java level) > >>> were to block (and therefore "take time" instead of being > >>> essentially > >> instantaneous). > >>> > >>> -chris > >>>> > >>>> ------------------------------------------------------------------- > >>>> -- To unsubscribe, e-mail: users-unsubscr...@tomcat.apache.org > >>>> For additional commands, e-mail: users-h...@tomcat.apache.org > >>> > >>> Disclaimer : This email and any files transmitted with it are > >> confidential and intended solely for intended recipients. 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