doc/smartsched

   1                         Client Scheduling in X
   2                             Keith Packard
   3                                SuSE
   4                              10/28/99
   5
   6 History:
   7
   8 Since the original X server was written at Digital in 1987, the OS and DIX
   9 layers shared responsibility for scheduling the order to service
  10 client requests.  The original design was simplistic; under the maximum
  11 first make it work, then make it work well, this was a good idea.  Now
  12 that we have a bit more experience with X applications, it's time to
  13 rethink the design.
  14
  15 The basic dispatch loop in DIX looks like:
  16
  17         for (;;)
  18         {
  19                 nready = WaitForSomething (...);
  20                 while (nready--)
  21                 {
  22                         isItTimeToYield = FALSE;
  23                         while (!isItTimeToYield)
  24                         {
  25                                 if (!ReadRequestFromClient (...))
  26                                         break;
  27                                 (execute request);
  28                         }
  29                 }
  30         }
  31
  32 WaitForSomething looks like:
  33
  34         for (;;)
  35                 if (ANYSET (ClientsWithInput))
  36                         return popcount (ClientsWithInput);
  37                 select (...)
  38                 compute clientsReadable from select result;
  39                 return popcount (clientsReadable)
  40         }
  41
  42 ReadRequestFromClient looks like:
  43
  44         if (!fullRequestQueued)
  45         {
  46                 read ();
  47                 if (!fullRequestQueued)
  48                 {
  49                         remove from ClientsWithInput;
  50                         timesThisConnection = 0;
  51                         return 0;
  52                 }
  53         }
  54         if (twoFullRequestsQueued)
  55                 add to ClientsWithInput;
  56
  57         if (++timesThisConnection >= 10)
  58         {
  59                 isItTimeToYield = TRUE;
  60                 timesThisConnection = 0;
  61         }
  62         return 1;
  63
  64 Here's what happens in this code:
  65
  66 With a single client executing a stream of requests:
  67
  68         A client sends a packet of requests to the server.
  69
  70         WaitForSomething wakes up from select and returns that client
  71         to Dispatch
  72
  73         Dispatch calls ReadRequestFromClient which reads a buffer (4K)
  74         full of requests from the client
  75
  76         The server executes requests from this buffer until it emptys,
  77         in two stages -- 10 requests at a time are executed in the
  78         inner Dispatch loop, a buffer full of requests are executed
  79         because WaitForSomething immediately returns if any clients
  80         have complete requests pending in their input queues.
  81
  82         When the buffer finally emptys, the next call to ReadRequest
  83         FromClient will return zero and Dispatch will go back to
  84         WaitForSomething; now that the client has no requests pending,
  85         WaitForSomething will block in select again.  If the client
  86         is active, this select will immediately return that client
  87         as ready to read.
  88
  89 With multiple clients sending streams of requests, the sequence
  90 of operations is similar, except that ReadRequestFromClient will
  91 set isItTimeToYield after each 10 requests executed causing the
  92 server to round-robin among the clients with available requests.
  93
  94 It's important to realize here that any complete requests which have been
  95 read from clients will be executed before the server will use select again
  96 to discover input from other clients.  A single busy client can easily
  97 monopolize the X server.
  98
  99 So, the X server doesn't share well with clients which are more interactive
 100 in nature.
 101
 102 The X server executes at most a buffer full of requests before again heading
 103 into select; ReadRequestFromClient causes the server to yield when the
 104 client request buffer doesn't contain a complete request.  When
 105 that buffer is executed quickly, the server spends a lot of time
 106 in select discovering that the same client again has input ready.  Thus
 107 the server also runs busy clients less efficiently than is would be
 108 possible.
 109
 110 What to do.
 111
 112 There are several things evident from the above discussion:
 113
 114  1      The server has a poor metric for deciding how much work it
 115         should do at one time on behalf of a particular client.
 116
 117  2      The server doesn't call select often enough to detect less
 118         aggressive clients in the face of busy clients, especially
 119         when those clients are executing slow requests.
 120
 121  3      The server calls select too often when executing fast requests.
 122
 123  4      Some priority scheme is needed to keep interactive clients
 124         responding to the user.
 125
 126 And, there are some assumptions about how X applications work:
 127
 128  1      Each X request is executed relatively quickly; a request-granularity
 129         is good enough for interactive response almost all of the time.
 130
 131  2      X applications receiving mouse/keyboard events are likely to
 132         warrant additional attention from the X server.
 133
 134 Instead of a request-count metric for work, a time-based metric should be
 135 used.  The server should select a reasonable time slice for each client
 136 and execute requests for the entire timeslice before yielding to
 137 another client.
 138
 139 Instead of returning immediately from WaitForSomething if clients have
 140 complete requests queued, the server should go through select each
 141 time and gather as many ready clients as possible.  This involves
 142 polling instead of blocking and adding the ClientsWithInput to
 143 clientsReadable after the select returns.
 144
 145 Instead of yielding when the request buffer is empty for a particular
 146 client, leave the yielding to the upper level scheduling and allow
 147 the server to try and read again from the socket.  If the client
 148 is busy, another buffer full of requests will already be waiting
 149 to be delivered thus avoiding the call through select and the
 150 additional overhead in WaitForSomething.
 151
 152 Finally, the dispatch loop should not simply execute requests from the
 153 first available client, instead each client should be prioritized with
 154 busy clients penalized and clients receiving user events praised.
 155
 156 How it's done:
 157
 158 Polling the current time of day from the OS is too expensive to
 159 be done at each request boundary, so instead an interval timer is
 160 set allowing the server to track time changes by counting invocations
 161 of the related signal handler.  Instead of using the wall time for
 162 this purpose, the process CPU time is used instead.  This serves
 163 two purposes -- first, it allows the server to consume no CPU cycles
 164 when idle, second it avoids conflicts with SIGALRM usage in other
 165 parts of the server code.  It's not without problems though; other
 166 CPU intensive processes on the same machine can reduce interactive
 167 response time within the X server.  The dispatch loop can now
 168 calculate an approximate time value using the number of signals
 169 received.  The granularity of the timer sets the scheduling jitter,
 170 at 20ms it's only occasionally noticeable.
 171
 172 The changes to WaitForSomething and ReadRequestFromClient are
 173 straightforward, adjusting when select is called and avoiding
 174 setting isItTimeToYield too often.
 175
 176 The dispatch loop changes are more extensive, now instead of
 177 executing requests from all available clients, a single client
 178 is chosen after each call to WaitForSomething, requests are
 179 executed for that client and WaitForSomething is called again.
 180
 181 Each client is assigned a priority, the dispatch loop chooses the
 182 client with the highest priority to execute.  Priorities are
 183 updated in three ways:
 184
 185  1.     Clients which consume their entire slice are penalized
 186         by having their priority reduced by one until they
 187         reach some minimum value.
 188
 189  2.     Clients which have executed no requests for some time
 190         are praised by having their priority raised until they
 191         return to normal priority.
 192
 193  3.     Clients which receive user input are praised by having
 194         their priority rased until they reach some maximal
 195         value, above normal priority.
 196
 197 The effect of these changes is to both improve interactive application
 198 response and benchmark numbers at the same time.
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 204 $XFree86: $