Re: [Qemu-devel] [PATCH for-2.10] throttle: Make LeakyBucket.avg and LeakyBucket.max integer types

[Eric Blake <eblake@redhat.com>]

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On 08/07/2017 11:15 AM, Alberto Garcia wrote:
> Both the throttling limits set with the throttling.iops-* and
> throttling.bps-* options and their QMP equivalents defined in the
> BlockIOThrottle struct are integer values.
> 
> Those limits are also reported in the BlockDeviceInfo struct and they
> are integers there as well.
> 
> Therefore there's no reason to store them internally as double and do
> the conversion everytime we're setting or querying them, so this patch
> uses int64_t for those types.
> 
> LeakyBucket.level and LeakyBucket.burst_level do however remain double
> because their value changes depending on the fraction of time elapsed
> since the previous I/O operation.
> 
> There's one particular instance of the previous code where bkt->max
> could have a non-integer value: that's in throttle_fix_bucket() when
> bkt->max is initialized to bkt->avg / 10. This is now an integer
> division and the result is rounded. We don't need to worry about this
> because:
> 
>    a) with the magnitudes we're dealing with (bytes per second, I/O
>       operations per second) the limits are likely to be always
>       multiples of 10.
> 
>    b) even if they weren't this doesn't affect the actual limits, only
>       the algorithm that makes the throttling smoother.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  include/qemu/throttle.h | 4 ++--
>  util/throttle.c         | 7 ++-----
>  2 files changed, 4 insertions(+), 7 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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