b7728f3221
It is possible for rate limited writes to keep overshooting a slice's quota by a tiny amount causing the slice-aligned waiting period to effectively halve the rate. Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com> Reviewed-by: Alberto Garcia <berto@igalia.com> Message-id: 20180207071758.6818-1-w.bumiller@proxmox.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
74 lines
2.2 KiB
C
74 lines
2.2 KiB
C
/*
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* Ratelimiting calculations
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*
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* Copyright IBM, Corp. 2011
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*
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* Authors:
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* Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
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*
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* This work is licensed under the terms of the GNU LGPL, version 2 or later.
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* See the COPYING.LIB file in the top-level directory.
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*
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*/
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#ifndef QEMU_RATELIMIT_H
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#define QEMU_RATELIMIT_H
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typedef struct {
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int64_t slice_start_time;
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int64_t slice_end_time;
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uint64_t slice_quota;
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uint64_t slice_ns;
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uint64_t dispatched;
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} RateLimit;
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/** Calculate and return delay for next request in ns
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*
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* Record that we sent @n data units (where @n matches the scale chosen
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* during ratelimit_set_speed). If we may send more data units
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* in the current time slice, return 0 (i.e. no delay). Otherwise
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* return the amount of time (in ns) until the start of the next time
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* slice that will permit sending the next chunk of data.
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*
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* Recording sent data units even after exceeding the quota is
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* permitted; the time slice will be extended accordingly.
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*/
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static inline int64_t ratelimit_calculate_delay(RateLimit *limit, uint64_t n)
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{
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int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
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double delay_slices;
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assert(limit->slice_quota && limit->slice_ns);
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if (limit->slice_end_time < now) {
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/* Previous, possibly extended, time slice finished; reset the
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* accounting. */
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limit->slice_start_time = now;
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limit->slice_end_time = now + limit->slice_ns;
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limit->dispatched = 0;
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}
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limit->dispatched += n;
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if (limit->dispatched < limit->slice_quota) {
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/* We may send further data within the current time slice, no
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* need to delay the next request. */
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return 0;
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}
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/* Quota exceeded. Wait based on the excess amount and then start a new
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* slice. */
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delay_slices = (double)limit->dispatched / limit->slice_quota;
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limit->slice_end_time = limit->slice_start_time +
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(uint64_t)(delay_slices * limit->slice_ns);
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return limit->slice_end_time - now;
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}
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static inline void ratelimit_set_speed(RateLimit *limit, uint64_t speed,
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uint64_t slice_ns)
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{
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limit->slice_ns = slice_ns;
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limit->slice_quota = MAX(((double)speed * slice_ns) / 1000000000ULL, 1);
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}
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#endif
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