-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Eric,
On 9/8/20 17:29, Eric Robinson wrote: > 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. Well... buffering can happen either during the client-write phase or the server-read phase or both. Imagine a slow network like EDGE or something similar where the first byte arrives at Tomcat's poller and it handed-off to the request-processor (t=0 as far as Tomcat is concerned) and uploads a large image over that EDGE connection. The OS won't allocate an infinite input buffer, so at some point the Poller will get byte 0 when the client hasn't uploaded the complete request. It may still take several seconds to upload all those bytes. Imagine that the response is a transformed image so the response is also large. The OS won't allocate an infinite output buffer, so at some point the bytes will start streaming to the client (at a slow rate). When the output buffer fills, your request-processing thread will stall when calling ServletOutputStream.write() to write those image bytes. If your image transform is instantaneous, your access log will report that the request took "a long time" relative to the amount of time spent actually processing the request. Basically, you are just waiting on I/O the entire time. Are you have any specific problem you are trying to diagnose or fix? Or are you just academically interested in what conditions might cause "slow" request processing? Hope that helps, - -chris >> -----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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