include/qemu/ratelimit.h

   1 /*
   2  * Ratelimiting calculations
   3  *
   4  * Copyright IBM, Corp. 2011
   5  *
   6  * Authors:
   7  *  Stefan Hajnoczi   <stefanha@linux.vnet.ibm.com>
   8  *
   9  * This work is licensed under the terms of the GNU LGPL, version 2 or later.
  10  * See the COPYING.LIB file in the top-level directory.
  11  *
  12  */
  13
  14 #ifndef QEMU_RATELIMIT_H
  15 #define QEMU_RATELIMIT_H
  16
  17 typedef struct {
  18     int64_t slice_start_time;
  19     int64_t slice_end_time;
  20     uint64_t slice_quota;
  21     uint64_t slice_ns;
  22     uint64_t dispatched;
  23 } RateLimit;
  24
  25 /** Calculate and return delay for next request in ns
  26  *
  27  * Record that we sent @n data units (where @n matches the scale chosen
  28  * during ratelimit_set_speed). If we may send more data units
  29  * in the current time slice, return 0 (i.e. no delay). Otherwise
  30  * return the amount of time (in ns) until the start of the next time
  31  * slice that will permit sending the next chunk of data.
  32  *
  33  * Recording sent data units even after exceeding the quota is
  34  * permitted; the time slice will be extended accordingly.
  35  */
  36 static inline int64_t ratelimit_calculate_delay(RateLimit *limit, uint64_t n)
  37 {
  38     int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
  39     double delay_slices;
  40
  41     assert(limit->slice_quota && limit->slice_ns);
  42
  43     if (limit->slice_end_time < now) {
  44         /* Previous, possibly extended, time slice finished; reset the
  45          * accounting. */
  46         limit->slice_start_time = now;
  47         limit->slice_end_time = now + limit->slice_ns;
  48         limit->dispatched = 0;
  49     }
  50
  51     limit->dispatched += n;
  52     if (limit->dispatched < limit->slice_quota) {
  53         /* We may send further data within the current time slice, no
  54          * need to delay the next request. */
  55         return 0;
  56     }
  57
  58     /* Quota exceeded. Wait based on the excess amount and then start a new
  59      * slice. */
  60     delay_slices = (double)limit->dispatched / limit->slice_quota;
  61     limit->slice_end_time = limit->slice_start_time +
  62         (uint64_t)(delay_slices * limit->slice_ns);
  63     return limit->slice_end_time - now;
  64 }
  65
  66 static inline void ratelimit_set_speed(RateLimit *limit, uint64_t speed,
  67                                        uint64_t slice_ns)
  68 {
  69     limit->slice_ns = slice_ns;
  70     limit->slice_quota = MAX(((double)speed * slice_ns) / 1000000000ULL, 1);
  71 }
  72
  73 #endif