[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-block] [PATCH 08/13] throttle: Add support for burst periods
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-block] [PATCH 08/13] throttle: Add support for burst periods |
Date: |
Tue, 16 Feb 2016 11:45:32 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 05.02.2016 um 11:59 hat Alberto Garcia geschrieben:
> This patch adds support for burst periods to the throttling code.
> With this feature the user can keep performing bursts as defined by
> the LeakyBucket.max rate for a configurable period of time.
>
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
> include/qemu/throttle.h | 41 +++++++++++++++++++++++----
> util/throttle.c | 73
> ++++++++++++++++++++++++++++++++++++++++---------
> 2 files changed, 96 insertions(+), 18 deletions(-)
>
> diff --git a/include/qemu/throttle.h b/include/qemu/throttle.h
> index 8ec8951..63df690 100644
> --- a/include/qemu/throttle.h
> +++ b/include/qemu/throttle.h
> @@ -2,7 +2,7 @@
> * QEMU throttling infrastructure
> *
> * Copyright (C) Nodalink, EURL. 2013-2014
> - * Copyright (C) Igalia, S.L. 2015
> + * Copyright (C) Igalia, S.L. 2015-2016
> *
> * Authors:
> * BenoƮt Canet <address@hidden>
> @@ -42,16 +42,47 @@ typedef enum {
> } BucketType;
>
> /*
> - * The max parameter of the leaky bucket throttling algorithm can be used to
> - * allow the guest to do bursts.
> - * The max value is a pool of I/O that the guest can use without being
> throttled
> - * at all. Throttling is triggered once this pool is empty.
> + * This module implements I/O limits using the leaky bucket
> + * algorithm. The code is independent of the I/O units, but it is
> + * currently used for bytes per second and operations per second.
> + *
> + * Three parameters can be set by the user:
> + *
> + * - avg: the desired I/O limits in units per second.
> + * - max: the limit during bursts, also in units per second.
> + * - burst_length: the maximum length of the burst period, in seconds.
> + *
> + * Here's how it works:
> + *
> + * - The bucket level (number of performed I/O units) is kept in
> + * bkt.level and leaks at a rate of bkt.avg units per second.
> + *
> + * - The size of the bucket is bkt.max * bkt.burst_length. Once the
> + * bucket is full no more I/O is performed until the bucket leaks
> + * again. This is what makes the I/O rate bkt.avg.
> + *
> + * - The bkt.avg rate does not apply until the bucket is full,
> + * allowing the user to do bursts until then. The I/O limit during
> + * bursts is bkt.max. To enforce this limit we keep an additional
> + * bucket in bkt.burst_length that leaks at a rate of bkt.max units
> + * per second.
> + *
> + * - Because of all of the above, the user can perform I/O at a
> + * maximum of bkt.max units per second for at most bkt.burst_length
> + * seconds in a row. After that the bucket will be full and the I/O
> + * rate will go down to bkt.avg.
> + *
> + * - Since the bucket always leaks at a rate of bkt.avg, this also
> + * determines how much the user needs to wait before being able to
> + * do bursts again.
> */
Good summary, thanks!
> typedef struct LeakyBucket {
> double avg; /* average goal in units per second */
> double max; /* leaky bucket max burst in units */
> double level; /* bucket level in units */
> + double burst_level; /* bucket level in units (for computing
> bursts) */
> + unsigned burst_length; /* max length of the burst period, in seconds
> */
> } LeakyBucket;
>
> /* The following structure is used to configure a ThrottleState
> diff --git a/util/throttle.c b/util/throttle.c
> index 6a01cee..371c769 100644
> --- a/util/throttle.c
> +++ b/util/throttle.c
> @@ -41,6 +41,14 @@ void throttle_leak_bucket(LeakyBucket *bkt, int64_t
> delta_ns)
>
> /* make the bucket leak */
> bkt->level = MAX(bkt->level - leak, 0);
> +
> + /* if we allow bursts for more than one second we also need to
> + * keep track of bkt->burst_level so the bkt->max goal per second
> + * is attained */
> + if (bkt->burst_length > 1) {
> + leak = (bkt->max * (double) delta_ns) / NANOSECONDS_PER_SECOND;
> + bkt->burst_level = MAX(bkt->burst_level - leak, 0);
> + }
> }
>
> /* Calculate the time delta since last leak and make proportionals leaks
> @@ -91,13 +99,24 @@ int64_t throttle_compute_wait(LeakyBucket *bkt)
> return 0;
> }
>
> - extra = bkt->level - bkt->max;
> + /* If the bucket is full then we have to wait */
> + extra = bkt->level - bkt->max * bkt->burst_length;
> + if (extra > 0) {
> + return throttle_do_compute_wait(bkt->avg, extra);
> + }
>
> - if (extra <= 0) {
> - return 0;
> + /* If the bucket is not full yet we have to make sure that we
> + * fulfill the goal of bkt->max units per second. */
> + if (bkt->burst_length > 1) {
> + /* We use 1/10 of the max value to smooth the throttling.
> + * See throttle_fix_bucket() for more details. */
> + extra = bkt->burst_level - bkt->max / 10;
I don't understand the connection between throttle_fix_bucket() and
this.
throttle_fix_bucket() seems to set a default rate for bursts, which kind
of makes sense to me (but what's the point when this is lower than the
average rate?)
Here we work on a bkt->max that is either supplied by the user and
should therefore be respected, or the default in throttle_fix_bucket()
has already been applied.
What this line does is letting the request wait for more than would be
strictly necessary, or in other words, for the last second before the
bucket runs full, you only allow a tenth of the actual maximum rate.
I understand that having any burst at all helps, so the default that
throttle_fix_bucket() sets used to make sense. I'm not so sure that it
still makes sense with its max < avg setting (max used to be additional
units on top of avg, now it's measured on its own). For the divison by
10 here, however, I'm still puzzled.
What am I missing?
> + if (extra > 0) {
> + return throttle_do_compute_wait(bkt->max, extra);
> + }
> }
>
> - return throttle_do_compute_wait(bkt->avg, extra);
> + return 0;
> }
Kevin
- [Qemu-block] [PATCH 00/13] throttle: Allow I/O bursts for a user-defined period of time, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 09/13] throttle: Add command-line settings to define the burst periods, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 12/13] throttle: Check that burst_level leaks correctly, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 04/13] throttle: Make throttle_is_valid() set errp, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 05/13] throttle: Set always an average value when setting a maximum value, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 01/13] throttle: Make throttle_compute_timer() static, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 11/13] qapi: Add burst length fields to BlockDeviceInfo, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 02/13] throttle: Make throttle_conflicting() set errp, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 07/13] throttle: Use throttle_config_init() to initialize ThrottleConfig, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 08/13] throttle: Add support for burst periods, Alberto Garcia, 2016/02/05
- Re: [Qemu-block] [PATCH 08/13] throttle: Add support for burst periods,
Kevin Wolf <=
- [Qemu-block] [PATCH 03/13] throttle: Make throttle_max_is_missing_limit() set errp, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 13/13] throttle: Test throttle_compute_wait() during bursts, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 06/13] throttle: Merge all functions that check the configuration into one, Alberto Garcia, 2016/02/05
- [Qemu-block] [PATCH 10/13] qapi: Add burst length parameters to block_set_io_throttle, Alberto Garcia, 2016/02/05
- Re: [Qemu-block] [PATCH 00/13] throttle: Allow I/O bursts for a user-defined period of time, Kevin Wolf, 2016/02/12
- Re: [Qemu-block] [PATCH 00/13] throttle: Allow I/O bursts for a user-defined period of time, Stefan Hajnoczi, 2016/02/15