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- <!-- #BeginEditable "Section%20title/logo" --><i><b><img src="./Inside IO Completion Ports_files/comport.gif" alt="Logo" width="445" height="29"><font face="arial" size="2"><a name="top"></a><br>
- </font></b></i><b>Copyright © 1998 <a href="mailto:mark@sysinternals.com">Mark
- Russinovich</a></b><!-- #EndEditable --></td>
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- </tbody></table>
- </td>
- </tr>
- <tr>
- <td colspan="2" valign="top">
- <table width="570" border="0" cellspacing="0" cellpadding="0">
- <tbody><tr valign="top">
- <td colspan="3" height="429"><!-- #BeginEditable "Body" -->
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- <tbody><tr valign="top" align="left">
- <td colspan="2" width="651" height="3148">
- <table width="580" border="0" cellspacing="0" cellpadding="0">
- <tbody><tr>
- <td valign="top">
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- <tbody><tr>
- <td colspan="3"><b></b></td>
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- <tr>
- <td colspan="3" valign="top"><span class="include">Last
- updated July 30, 1998</span>
- <hr>
- </td>
- </tr>
- <tr>
- <td colspan="3" height="40" valign="middle"><span class="sectionheader">Introduction</span>
- </td>
- </tr>
- <tr>
- <td valign="TOP" colspan="3" align="left"><font color="#000000">Writing
- a high-performance server application requires implementing
- an efficient threading model. Having either too
- few or too many server threads to process client
- requests can lead to performance problems. For example,
- if a server creates a single thread to handle all
- requests clients can become starved since the server
- will be tied up processing one request at a time.
- Of course, a single thread could simultaneously
- process multiple requests, switching from one to
- another as I/O operations are started, but this
- architecture introduces significant complexity and
- cannot take advantage of multiprocessor systems.
- At the other extreme a server could create a big
- pool of threads so that virtually every client request
- is processed by a dedicated thread. This scenario
- usually leads to thread-thrashing, where lots of
- threads wake-up, perform some CPU processing, block
- waiting for I/O and then after request procesing
- is completed block again waiting for a new request.
- If nothing else, context-switches are caused by
- the scheduler having to divide processor time among
- multiple active threads.</font><br>
- <br>
- <font color="#000000">The goal of a server is to
- incur as few context switches as possible by having
- its threads avoid unnecessary blocking, while at
- the same time maximizing parallelism by using multiple
- threads. The ideal is for there to be a thread actively
- servicing a client request on every processor and
- for those threads not to block if there are additional
- requests waiting when they complete a request. For
- this to work correctly however, there must be a
- way for the application to activate another thread
- when one processing a client request blocks on I/O
- (like when it reads from a file as part of the processing).
- </font><br>
- <br>
- <font color="#000000">Windows NT 3.5 introduced
- a set of APIs that make this goal relatively easy
- to achieve. The APIs are centered on an object called
- a <i>completion port</i>. In this article I'm going
- to provide an overview of how completion ports are
- used and then go inside them to show you how Windows
- NT implements them. </font>
- </td>
- </tr>
- <tr>
- <td colspan="3" height="40" valign="middle"><span class="sectionheader">Using I/O Completion Ports </span></td>
- </tr>
- <tr>
- <td valign="TOP" colspan="3">Applications use completion
- ports as the the focal point for the completion
- of I/O associated with multiple file handles. Once
- a file is associated with a completion port any
- asynchronous I/O operations that complete on the
- file result in a completion packet being queued
- to the port. A thread can wait for any outstanding
- I/Os to complete on multiple files simply by waiting
- for a completion packet to be queued on the completion
- port. The Win32 API provides similar functionality
- with the <b>WaitForMultipleObjects</b> API, but
- the advantage that completion ports have is that
- concurrency, or the number of threads that an application
- has actively servicing client requests, is controlled
- with the aid of the system. <br>
- <br>
- When an application creates a completion port it
- specifies a concurrency value. This value indicates
- the maximum number of threads associated with the
- port that should be running at any given point in
- time. As I stated earlier, the ideal is to have
- one thread active at any given point in time for
- every processor in the system. The concurrency value
- associated with a port is used by NT to control
- how many threads an application has active - if
- the number of active threads associated with a port
- equals the concurrency value then a thread that
- is waiting on the completion port will not be allowed
- to run. Instead, it is expected that one of the
- active threads will finish processing its current
- request and check to see if there's another packet
- waiting at the port - if there is then it simply
- grabs it and goes off to process it. When this happens
- there is no context switch, and the CPUs are utilized
- to near their full capacity. <br>
- <br>
- Figure 1 below shows a high-level picture of completion
- port operation. Incoming client requests cause completion
- packets to be queued at the port. A number of threads,
- up to the concurrency limit for the port, are allowed
- by NT to process client requests. Any additional
- threads associated with the port are blocked until
- the number of active threads drops, as can happen
- when an active thread blocks on file I/O. I'll discuss
- this further a little later.<br><br>
- <img src="./Inside IO Completion Ports_files/comport1.gif" width="529" height="326"><br>
- A completion port is created with a call to the
- Win32 API <b>CreateIoCompletionPort</b>: <br>
- <p><font color="#000000"><b>HANDLE CreateIoCompletionPort(
- </b> <br>
- <b> HANDLE</b>
- <i>FileHandle</i>,<b><br>
- HANDLE</b>
- <i>ExistingCompletionPort</i>, <br>
- <b> DWORD</b>
- <i>CompletionKey ,</i><b><br>
- DWORD</b>
- <i>NumberOfConcurrentThreads</i><b><br>
- );</b>
- </font></p>
- <br>
- To create the port an application passes in a NULL
- for the <i>ExistingCompletionPort</i> parameter
- and indicates the concurreny value with the <i>NumberOfConcurrentThreads</i>
- parameter. If a <i>FileHandle</i> parameter is specified
- then the file handle becomes associated with the
- port. When an I/O request that has been issued on
- the file handle completes a completion packet is
- queued to the completion port. To retrieve a completion
- packet and possibly block waiting for one to arrive
- a thread calls the <b>GetQueuedCompletionStatus</b>
- API: <br>
- <p><font color="#000000"><b>BOOL GetQueuedCompletionStatus(</b><br>
- <b> HANDLE</b>
- <i>CompletionPort</i>,<b><br>
- LPDWORD</b>
- <i>lpNumberOfBytesTransferred</i>, <br>
- <b> LPDWORD</b>
- <i>CompletionKey ,</i><b><br>
- LPOVERLAPPED</b>
- <i>*lpOverlapped,</i><b><br>
- DWORD</b>
- <i>dwMiillisecondTimeout</i><b><br>
- );</b>
- </font></p>
- <br>
- Threads that block on a completion port become associated
- with the port and are woken in LIFO order so that
- the thread that blocked most recently is the one
- that is given the next packet. Threads that block
- for long periods of time can have their stacks swapped
- out to disk, so if there are more threads associated
- with a port then there is work to process the in-memory
- footprints of threads blocked the longest are minimized.
- <br>
- <br>
- A server application will usually receive client
- requests via network endpoints that are represented
- as file handles. Examples include Winsock2 sockets
- or named pipes. As the server creates its communications
- endpoints it associates them with a completion port
- and its threads wait for incoming requests by calling
- <b>GetQueuedCompletionStatus</b> on the port. When
- a thread is given a packet from the completion port
- it will go off and start processing the request,
- becoming an active thread. Many times a thread will
- block during its processing, like when it needs
- to read or write data to a file on disk, or when
- it synchronizes with other threads. Windows NT is
- clever enough to detect this and recognize that
- the completion port has one less active thread.
- Therefore, when a thread becomes inactive because
- it blocks, a thread waiting on the completion port
- will be woken if there is packet in the queue. <br>
- <br>
- Microsoft's guidelines are to set the concurrency
- value roughly equal to the number of processors
- in a system. Note that it is possible for the number
- of active threads for a completion port to exceed
- the concurrency limit. Consider a case where the
- limit is specified as 1. A client request comes
- in and a thread is dispatched to process the request,
- becoming active. A second requests comes in but
- a second thread waiting on the port is not allowed
- to proceed because the concurrency limit has been
- reached. Then the first thread blocks waiting for
- a file I/O so it becomes inactive. The second thread
- is then released and while it is still active the
- first thread's file I/O is completes, making it
- active again. At that point in time, and until one
- of the threads blocks, the concurrency value is
- 2, which is higher than the limit of 1. Most of
- the time the active count will remain at or just
- above the concurrency limit.<br>
- <br>
- The completion port API also makes it possible for
- a server application to queue privately defined
- completion packets to a completion port using <b>PostQueuedCompletionStatus</b>.
- Servers typically use this function to inform its
- threads of external events such as the need to shut
- down gracefully.</td>
- </tr>
- <tr>
- <td height="40" colspan="3" valign="middle"><span class="sectionheader">Completion Port Internals</span>
- </td>
- </tr>
- <tr>
- <td colspan="3" valign="TOP">
- <p><font color="#000000">A call to the Win32 API
- <b>CreateIoCompletionPort</b> with a NULL completion
- port handle results in the execution of the native
- API function <b>NtCreateIoCompletion</b>, which
- invokes the corresponding kernel-mode system service
- of the same name. Internally, completion ports
- are based on an undocumented executive synchronization
- object called a <i>Queue</i>. Thus, the system
- service creates a completion port object and initializes
- a queue object in the port's allocated memory
- (a pointer to the port also points to the queue
- object since the queue is at the start of the
- port memory). A queue object has (coincidentally)
- a concurrency value that is specified when a thread
- initializes one, and in this case the value that
- is used is the one that was passed to <b>CreateIoCompletionPort</b>.
- <b>KeInitializeQueue</b> is the function that
- <b>NtCreateIoCompletion</b> calls to initialize
- a port's queue object.</font> <br>
- <br>
- <font color="#000000">When an application calls
- <b>CreateIoCompletionPort</b> to associate a file
- handle with a port the Win32 API invokes the native
- function <b>NtSetInformationFile</b> with the
- file handle as the primary parameter. The information
- class that is set is <b>FileCompletionInformation</b>
- and the completion port's handle and the <i>CompletionKey</i>
- parameter from <b>CreateIoCompletionPort</b> are
- the data values. <b>NtSetInformationFile</b> dereferences
- the file handle to obtain the file object and
- allocates a completion context data structure,
- which is defined in NTDDK.H as: </font><br>
- </p>
- <p><font color="#000000"><b>typedef struct _IO_COMPLETION_CONTEXT
- {</b><br>
- <b> PVOID</b>
- <i>Port</i>;<b> <br>
- ULONG</b>
- <i>Key</i>; <br>
- <b>} IO_COMPLETION_CONTEXT, *PIO_COMPLETION_CONTEXT;</b></font></p>
- <br>
- <font color="#000000">Finally, <b>NtSetInformationFile</b>
- sets the <i>CompletionContext</i> field in the file
- object to point at the context structure. When an
- I/O operation completes on a file object the internal
- I/O manager function <b>IopCompleteRequest</b> executes
- and, if the I/O was asynchronous, checks to see
- if the <i>CompletionContext</i> field in the file
- object is non-NULL. If its non-NULL the I/O Manager
- allocates a completion packet and queues it to the
- completion port by calling <b>KeInsertQueue</b>
- with the port as the queue on which to insert the
- packet (remember that the completion port object
- and queue object are synonymous).</font><br>
- <br>
- <font color="#000000">When <b>GetQueuedCompletionStatus</b>
- is invoked by a server thread, it calls the native
- API function <b>NtRemoveIoCompletion</b>, which
- transfers control to the <b>NtRemoveIoCompletion</b>
- system service. After validating parameters and
- translating the completion port handle to a pointer
- to the port, <b>NtRemoveIoCompletion</b> calls <b>KeRemoveQueue</b>.
- </font><br>
- <br>
- <font color="#000000">As you can see, <b>KeRemoveQueue</b>
- and <b>KeInsertQueue</b> are the engine behind completion
- ports and are the functions that determine whether
- a thread waiting for an I/O completion packet should
- be activated or not. Internally, a queue object
- maintains a count of the current number of active
- threads and the maximum active threads. If the current
- number equals or exceeds the maximum when a thread
- calls <b>KeRemoveQueue</b>, the thread will be put
- (in LIFO order) onto a list of threads waiting for
- a turn to process a completion packet. The list
- of threads hangs off the queue object. A thread's
- control block data structure has a pointer in it
- that references the queue object of a queue that
- it is associated with; if the pointer is NULL then
- the thread is not associated with a queue. </font><br>
- <br>
- <font color="#000000">So how does NT keep track
- of threads that become inactive because they block
- on something other than the completion port? The
- answer lies in the queue pointer in a thread's control
- block. The scheduler routines that are executed
- in response to a thread blocking (<b>KeWaitForSingleObject</b>,
- <b>KeDelayExecutionThread</b>, etc.) check the thread's
- queue pointer and if its not NULL they will call
- <b>KiActivateWaiterQueue</b>, a queue-related function.
- <b>KiActivateWaiterQueue</b> decrements the count
- of active threads associated with the queue, and
- if the result is less than the maximum and there
- is at least one completion packet in the queue then
- the thread at the front of the queue's thread list
- is woken and given the oldest packet. Conversely,
- whenever a thread that is associated with a queue
- wakes up after blocking the scheduler executes the
- function <b>KiUnwaitThread</b>, which increments
- the queue's active count. </font><br>
- <br>
- Finally, the <b>PostQueuedCompletionStatus</b> Win32
- API calls upon the native function <b>NtSetIoCompletion</b>.
- As with the other native APIs in the completion
- port group, this one invokes a system service bearing
- the same name, which simply inserts that packet
- onto the completion port's queue using <b>KeInsertQueue</b>.</td>
- </tr>
- <tr>
- <td colspan="3" height="40" valign="middle"><span class="sectionheader">Not
- Exported</span></td>
- </tr>
- <tr>
- <td valign="TOP" colspan="3" align="left"> Windows
- NT's completion port API provides an easy-to-use
- and efficient way to maximize a server's performance
- by minimizing context switches while obtaining high-degrees
- of parallelism. The API is made possible with support
- in the I/O Manager, Kernel, and system services.
- While the Queue object is exported for use by device
- drivers (it is undocumented but its interfaces are
- relatively easy to figure out), the completion port
- APIs are not. However, if the queue interfaces are
- derived it is possible to mimick the completion
- port interfaces by simply using the queue routines
- and manually associating file objects with queues
- by setting the <i>CompletionContext</i> entry.</td>
- </tr>
- <tr>
- <td colspan="3" height="40" valign="middle" align="center">
- <a href="https://web.archive.org/web/20101031075704/http://doc.sch130.nsc.ru/www.sysinternals.com/ntw2k/info/comport.shtml#top"><b>Back to Top</b></a> </td>
- </tr>
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