diff options
Diffstat (limited to 'xorg-server/doc/smartsched')
| -rw-r--r-- | xorg-server/doc/smartsched | 408 |
1 files changed, 204 insertions, 204 deletions
diff --git a/xorg-server/doc/smartsched b/xorg-server/doc/smartsched index 057a759fd..466408431 100644 --- a/xorg-server/doc/smartsched +++ b/xorg-server/doc/smartsched @@ -1,204 +1,204 @@ - Client Scheduling in X - Keith Packard - SuSE - 10/28/99 - -History: - -Since the original X server was written at Digital in 1987, the OS and DIX -layers shared responsibility for scheduling the order to service -client requests. The original design was simplistic; under the maximum -first make it work, then make it work well, this was a good idea. Now -that we have a bit more experience with X applications, it's time to -rethink the design. - -The basic dispatch loop in DIX looks like: - - for (;;) - { - nready = WaitForSomething (...); - while (nready--) - { - isItTimeToYield = FALSE; - while (!isItTimeToYield) - { - if (!ReadRequestFromClient (...)) - break; - (execute request); - } - } - } - -WaitForSomething looks like: - - for (;;) - if (ANYSET (ClientsWithInput)) - return popcount (ClientsWithInput); - select (...) - compute clientsReadable from select result; - return popcount (clientsReadable) - } - -ReadRequestFromClient looks like: - - if (!fullRequestQueued) - { - read (); - if (!fullRequestQueued) - { - remove from ClientsWithInput; - timesThisConnection = 0; - return 0; - } - } - if (twoFullRequestsQueued) - add to ClientsWithInput; - - if (++timesThisConnection >= 10) - { - isItTimeToYield = TRUE; - timesThisConnection = 0; - } - return 1; - -Here's what happens in this code: - -With a single client executing a stream of requests: - - A client sends a packet of requests to the server. - - WaitForSomething wakes up from select and returns that client - to Dispatch - - Dispatch calls ReadRequestFromClient which reads a buffer (4K) - full of requests from the client - - The server executes requests from this buffer until it emptys, - in two stages -- 10 requests at a time are executed in the - inner Dispatch loop, a buffer full of requests are executed - because WaitForSomething immediately returns if any clients - have complete requests pending in their input queues. - - When the buffer finally emptys, the next call to ReadRequest - FromClient will return zero and Dispatch will go back to - WaitForSomething; now that the client has no requests pending, - WaitForSomething will block in select again. If the client - is active, this select will immediately return that client - as ready to read. - -With multiple clients sending streams of requests, the sequence -of operations is similar, except that ReadRequestFromClient will -set isItTimeToYield after each 10 requests executed causing the -server to round-robin among the clients with available requests. - -It's important to realize here that any complete requests which have been -read from clients will be executed before the server will use select again -to discover input from other clients. A single busy client can easily -monopolize the X server. - -So, the X server doesn't share well with clients which are more interactive -in nature. - -The X server executes at most a buffer full of requests before again heading -into select; ReadRequestFromClient causes the server to yield when the -client request buffer doesn't contain a complete request. When -that buffer is executed quickly, the server spends a lot of time -in select discovering that the same client again has input ready. Thus -the server also runs busy clients less efficiently than is would be -possible. - -What to do. - -There are several things evident from the above discussion: - - 1 The server has a poor metric for deciding how much work it - should do at one time on behalf of a particular client. - - 2 The server doesn't call select often enough to detect less - aggressive clients in the face of busy clients, especially - when those clients are executing slow requests. - - 3 The server calls select too often when executing fast requests. - - 4 Some priority scheme is needed to keep interactive clients - responding to the user. - -And, there are some assumptions about how X applications work: - - 1 Each X request is executed relatively quickly; a request-granularity - is good enough for interactive response almost all of the time. - - 2 X applications receiving mouse/keyboard events are likely to - warrant additional attention from the X server. - -Instead of a request-count metric for work, a time-based metric should be -used. The server should select a reasonable time slice for each client -and execute requests for the entire timeslice before yielding to -another client. - -Instead of returning immediately from WaitForSomething if clients have -complete requests queued, the server should go through select each -time and gather as many ready clients as possible. This involves -polling instead of blocking and adding the ClientsWithInput to -clientsReadable after the select returns. - -Instead of yielding when the request buffer is empty for a particular -client, leave the yielding to the upper level scheduling and allow -the server to try and read again from the socket. If the client -is busy, another buffer full of requests will already be waiting -to be delivered thus avoiding the call through select and the -additional overhead in WaitForSomething. - -Finally, the dispatch loop should not simply execute requests from the -first available client, instead each client should be prioritized with -busy clients penalized and clients receiving user events praised. - -How it's done: - -Polling the current time of day from the OS is too expensive to -be done at each request boundary, so instead an interval timer is -set allowing the server to track time changes by counting invocations -of the related signal handler. Instead of using the wall time for -this purpose, the process CPU time is used instead. This serves -two purposes -- first, it allows the server to consume no CPU cycles -when idle, second it avoids conflicts with SIGALRM usage in other -parts of the server code. It's not without problems though; other -CPU intensive processes on the same machine can reduce interactive -response time within the X server. The dispatch loop can now -calculate an approximate time value using the number of signals -received. The granularity of the timer sets the scheduling jitter, -at 20ms it's only occasionally noticeable. - -The changes to WaitForSomething and ReadRequestFromClient are -straightforward, adjusting when select is called and avoiding -setting isItTimeToYield too often. - -The dispatch loop changes are more extensive, now instead of -executing requests from all available clients, a single client -is chosen after each call to WaitForSomething, requests are -executed for that client and WaitForSomething is called again. - -Each client is assigned a priority, the dispatch loop chooses the -client with the highest priority to execute. Priorities are -updated in three ways: - - 1. Clients which consume their entire slice are penalized - by having their priority reduced by one until they - reach some minimum value. - - 2. Clients which have executed no requests for some time - are praised by having their priority raised until they - return to normal priority. - - 3. Clients which receive user input are praised by having - their priority rased until they reach some maximal - value, above normal priority. - -The effect of these changes is to both improve interactive application -response and benchmark numbers at the same time. - - - - - -$XFree86: $ + Client Scheduling in X
+ Keith Packard
+ SuSE
+ 10/28/99
+
+History:
+
+Since the original X server was written at Digital in 1987, the OS and DIX
+layers shared responsibility for scheduling the order to service
+client requests. The original design was simplistic; under the maximum
+first make it work, then make it work well, this was a good idea. Now
+that we have a bit more experience with X applications, it's time to
+rethink the design.
+
+The basic dispatch loop in DIX looks like:
+
+ for (;;)
+ {
+ nready = WaitForSomething (...);
+ while (nready--)
+ {
+ isItTimeToYield = FALSE;
+ while (!isItTimeToYield)
+ {
+ if (!ReadRequestFromClient (...))
+ break;
+ (execute request);
+ }
+ }
+ }
+
+WaitForSomething looks like:
+
+ for (;;)
+ if (ANYSET (ClientsWithInput))
+ return popcount (ClientsWithInput);
+ select (...)
+ compute clientsReadable from select result;
+ return popcount (clientsReadable)
+ }
+
+ReadRequestFromClient looks like:
+
+ if (!fullRequestQueued)
+ {
+ read ();
+ if (!fullRequestQueued)
+ {
+ remove from ClientsWithInput;
+ timesThisConnection = 0;
+ return 0;
+ }
+ }
+ if (twoFullRequestsQueued)
+ add to ClientsWithInput;
+
+ if (++timesThisConnection >= 10)
+ {
+ isItTimeToYield = TRUE;
+ timesThisConnection = 0;
+ }
+ return 1;
+
+Here's what happens in this code:
+
+With a single client executing a stream of requests:
+
+ A client sends a packet of requests to the server.
+
+ WaitForSomething wakes up from select and returns that client
+ to Dispatch
+
+ Dispatch calls ReadRequestFromClient which reads a buffer (4K)
+ full of requests from the client
+
+ The server executes requests from this buffer until it emptys,
+ in two stages -- 10 requests at a time are executed in the
+ inner Dispatch loop, a buffer full of requests are executed
+ because WaitForSomething immediately returns if any clients
+ have complete requests pending in their input queues.
+
+ When the buffer finally emptys, the next call to ReadRequest
+ FromClient will return zero and Dispatch will go back to
+ WaitForSomething; now that the client has no requests pending,
+ WaitForSomething will block in select again. If the client
+ is active, this select will immediately return that client
+ as ready to read.
+
+With multiple clients sending streams of requests, the sequence
+of operations is similar, except that ReadRequestFromClient will
+set isItTimeToYield after each 10 requests executed causing the
+server to round-robin among the clients with available requests.
+
+It's important to realize here that any complete requests which have been
+read from clients will be executed before the server will use select again
+to discover input from other clients. A single busy client can easily
+monopolize the X server.
+
+So, the X server doesn't share well with clients which are more interactive
+in nature.
+
+The X server executes at most a buffer full of requests before again heading
+into select; ReadRequestFromClient causes the server to yield when the
+client request buffer doesn't contain a complete request. When
+that buffer is executed quickly, the server spends a lot of time
+in select discovering that the same client again has input ready. Thus
+the server also runs busy clients less efficiently than is would be
+possible.
+
+What to do.
+
+There are several things evident from the above discussion:
+
+ 1 The server has a poor metric for deciding how much work it
+ should do at one time on behalf of a particular client.
+
+ 2 The server doesn't call select often enough to detect less
+ aggressive clients in the face of busy clients, especially
+ when those clients are executing slow requests.
+
+ 3 The server calls select too often when executing fast requests.
+
+ 4 Some priority scheme is needed to keep interactive clients
+ responding to the user.
+
+And, there are some assumptions about how X applications work:
+
+ 1 Each X request is executed relatively quickly; a request-granularity
+ is good enough for interactive response almost all of the time.
+
+ 2 X applications receiving mouse/keyboard events are likely to
+ warrant additional attention from the X server.
+
+Instead of a request-count metric for work, a time-based metric should be
+used. The server should select a reasonable time slice for each client
+and execute requests for the entire timeslice before yielding to
+another client.
+
+Instead of returning immediately from WaitForSomething if clients have
+complete requests queued, the server should go through select each
+time and gather as many ready clients as possible. This involves
+polling instead of blocking and adding the ClientsWithInput to
+clientsReadable after the select returns.
+
+Instead of yielding when the request buffer is empty for a particular
+client, leave the yielding to the upper level scheduling and allow
+the server to try and read again from the socket. If the client
+is busy, another buffer full of requests will already be waiting
+to be delivered thus avoiding the call through select and the
+additional overhead in WaitForSomething.
+
+Finally, the dispatch loop should not simply execute requests from the
+first available client, instead each client should be prioritized with
+busy clients penalized and clients receiving user events praised.
+
+How it's done:
+
+Polling the current time of day from the OS is too expensive to
+be done at each request boundary, so instead an interval timer is
+set allowing the server to track time changes by counting invocations
+of the related signal handler. Instead of using the wall time for
+this purpose, the process CPU time is used instead. This serves
+two purposes -- first, it allows the server to consume no CPU cycles
+when idle, second it avoids conflicts with SIGALRM usage in other
+parts of the server code. It's not without problems though; other
+CPU intensive processes on the same machine can reduce interactive
+response time within the X server. The dispatch loop can now
+calculate an approximate time value using the number of signals
+received. The granularity of the timer sets the scheduling jitter,
+at 20ms it's only occasionally noticeable.
+
+The changes to WaitForSomething and ReadRequestFromClient are
+straightforward, adjusting when select is called and avoiding
+setting isItTimeToYield too often.
+
+The dispatch loop changes are more extensive, now instead of
+executing requests from all available clients, a single client
+is chosen after each call to WaitForSomething, requests are
+executed for that client and WaitForSomething is called again.
+
+Each client is assigned a priority, the dispatch loop chooses the
+client with the highest priority to execute. Priorities are
+updated in three ways:
+
+ 1. Clients which consume their entire slice are penalized
+ by having their priority reduced by one until they
+ reach some minimum value.
+
+ 2. Clients which have executed no requests for some time
+ are praised by having their priority raised until they
+ return to normal priority.
+
+ 3. Clients which receive user input are praised by having
+ their priority rased until they reach some maximal
+ value, above normal priority.
+
+The effect of these changes is to both improve interactive application
+response and benchmark numbers at the same time.
+
+
+
+
+
+$XFree86: $
|