Understanding Application Performance on the Network | Part 5
Jul. 14, 2014 08:45 AM
In Part IV, we wrapped up our discussions on bandwidth, congestion and packet loss. In Part V, we examine the four types of processing delays visible on the network, using the request/reply paradigm we outlined in Part I.
Server Processing (Between Flows)
From the network's perspective, we allocate the time period between the end of a request flow and the beginning of the corresponding reply flow to server processing. Generally speaking, the server doesn't begin processing a request until it has received the entire flow, i.e., the last packet in the request message; similarly, the server doesn't begin sending the reply until it has finished processing the request. We sometimes refer to these delays between flows as "pure" processing delays, distinct from another type of intra-flow processing delay we call starved for data and discuss later. Server processing delays occur as a result of a request message, and therefore always occur within a thread.
Transaction Trace Illustrations
These "pure" server processing delays are generally relatively simple to detect, to understand, and to prove. Transaction Trace's Node Processing table lists all of the observed processing delays for an operation in tabular format; by splitting this table with the Bounce Diagram and highlighting a row of interest, the Bounce Diagram will display the last packet of the request flow and the first packet of the corresponding reply flow, effectively diagramming the measurement.
Use the Node Processing Table split with the Bounce Diagram to illustrate node processing delay
You may also use the Thread Analysis split with the Bounce Diagram; this will provide a view of the request and reply packet flows as well as the processing measurement.
Split the Thread View with the Bounce Diagram to illustrate the request flow, node processing and reply flow
Starved for Data (Sending Node, Within a Flow)
Sometimes, the network interface will be able to transmit data at a rate faster than the sending application can deliver to the TCP socket. For example, a busy ftp server may momentarily interrupt sending a large file because of a disk, memory or CPU bottleneck. We refer to these pauses that occur in the middle of a flow as "starved for data" conditions; there is nothing on the network (no TCP flow control constraint) preventing the request or reply flow from continuing, so the cause must be internal to the sending node. Starved for data bottlenecks occur within a flow (instead of between flows), and are related to the sending node - either the client or server.
Transaction Trace Illustration
These cases can be more difficult to visualize. Since the condition is generally not too common, it is often best to rule out other performance bottlenecks first, before checking for data starvation. When it does occur, the condition has the effect of extending the duration of a request or reply flow, and starved for data delays are included in Transaction Trace's Node Sending measurements. Sort the rate column of the Node Sending Table and split the window with the Bounce Diagram; the Bounce Diagram will illustrate the packets associated with a sending measurement. For those sending measurements where you suspect a starved for data condition, look for idle periods of time where the sending node's flow has been interrupted. Importantly, a starved for data delay will terminate with the transmission of a data packet that resumes the sender's flow, not a TCP ACK from the receiver that might suggest a TCP or application window constraint.
The Node Sending table is split with the Bounce Diagram to help illustrate Starved for Data conditions; note the pause in transmission that resumes independent of any TCP ACK.
Client Processing (Between Flows)
From the network's perspective, we allocate the time period between the end of a reply flow and the beginning of the next request flow to client processing. Generally speaking, the client cannot begin processing a reply until it has received the entire reply flow, i.e., the last packet of the reply message; similarly, the client doesn't begin sending the next (new) request until it has completed processing the reply. (This correlation generally applies to request/reply flows on the same TCP connection.)
Transaction Trace Illustrations
Similar to server processing delays, client delays are relatively simple to understand. In most cases, client delays occur between threads; in other words, after one thread has completed but before the next thread begins. For tasks with thread-level decodes, Transaction Trace's Thread Analysis Gantt chart view can illustrate these delays well.
Gaps between threads are associated with client processing delays
Discounting Client Processing
Note that we assume strict adherence to the definition of "operation" here; click to screen update. If the trace has captured multiple steps - say the user navigates through a series of operations - then the user "think time" between steps will appear as client processing delay, with corresponding gaps between threads. You may still use these multi-step tasks for analysis, remembering to discount client processing delays. Alternatively, you can save each step as a separate task by selecting a sequence of threads from the Thread table.
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