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 #include "qemu/timer.h"
  18
  19 typedef struct {
  20     int64_t slice_start_time;
  21     int64_t slice_end_time;
  22     uint64_t slice_quota;
  23     uint64_t slice_ns;
  24     uint64_t dispatched;
  25 } RateLimit;
  26
  27 /** Calculate and return delay for next request in ns
  28  *
  29  * Record that we sent @n data units (where @n matches the scale chosen
  30  * during ratelimit_set_speed). If we may send more data units
  31  * in the current time slice, return 0 (i.e. no delay). Otherwise
  32  * return the amount of time (in ns) until the start of the next time
  33  * slice that will permit sending the next chunk of data.
  34  *
  35  * Recording sent data units even after exceeding the quota is
  36  * permitted; the time slice will be extended accordingly.
  37  */
  38 static inline int64_t ratelimit_calculate_delay(RateLimit *limit, uint64_t n)
  39 {
  40     int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
  41     double delay_slices;
  42
  43     assert(limit->slice_quota && limit->slice_ns);
  44
  45     if (limit->slice_end_time < now) {
  46         /* Previous, possibly extended, time slice finished; reset the
  47          * accounting. */
  48         limit->slice_start_time = now;
  49         limit->slice_end_time = now + limit->slice_ns;
  50         limit->dispatched = 0;
  51     }
  52
  53     limit->dispatched += n;
  54     if (limit->dispatched < limit->slice_quota) {
  55         /* We may send further data within the current time slice, no
  56          * need to delay the next request. */
  57         return 0;
  58     }
  59
  60     /* Quota exceeded. Wait based on the excess amount and then start a new
  61      * slice. */
  62     delay_slices = (double)limit->dispatched / limit->slice_quota;
  63     limit->slice_end_time = limit->slice_start_time +
  64         (uint64_t)(delay_slices * limit->slice_ns);
  65     return limit->slice_end_time - now;
  66 }
  67
  68 static inline void ratelimit_set_speed(RateLimit *limit, uint64_t speed,
  69                                        uint64_t slice_ns)
  70 {
  71     limit->slice_ns = slice_ns;
  72     limit->slice_quota = MAX(((double)speed * slice_ns) / 1000000000ULL, 1);
  73 }
  74
  75 #endif