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Documentation: kvm: clarify histogram units

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In the case of histogram statistics, the values are always sample
counts; the unit instead applies to the bucket range.  For example,
halt_poll_success_hist is a nanosecond statistic because the buckets are
for 0ns, 1ns, 2-3ns, 4-7ns etc.  There isn't really any other sensible
interpretation, but clarify this anyway in the Documentation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2022-07-14 07:29:57 -04:00
parent 1b870fa557
commit 942d9e8952
1 changed files with 7 additions and 4 deletions
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Documentation/virt/kvm/api.rst
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@ -5703,14 +5703,13 @@ Bits 0-3 of ``flags`` encode the type:
by the ``hist_param`` field. The range of the Nth bucket (1 <= N < ``size``)
is [``hist_param``*(N-1), ``hist_param``*N), while the range of the last
bucket is [``hist_param``*(``size``-1), +INF). (+INF means positive infinity
value.) The bucket value indicates how many samples fell in the bucket's range.
value.)
* ``KVM_STATS_TYPE_LOG_HIST``
The statistic is reported as a logarithmic histogram. The number of
buckets is specified by the ``size`` field. The range of the first bucket is
[0, 1), while the range of the last bucket is [pow(2, ``size``-2), +INF).
Otherwise, The Nth bucket (1 < N < ``size``) covers
[pow(2, N-2), pow(2, N-1)). The bucket value indicates how many samples fell
in the bucket's range.
[pow(2, N-2), pow(2, N-1)).
Bits 4-7 of ``flags`` encode the unit:
@ -5731,6 +5730,10 @@ Bits 4-7 of ``flags`` encode the unit:
statistics can be linear histograms (with two buckets) but not logarithmic
histograms.
Note that, in the case of histograms, the unit applies to the bucket
ranges, while the bucket value indicates how many samples fell in the
bucket's range.
Bits 8-11 of ``flags``, together with ``exponent``, encode the scale of the
unit:
@ -5752,7 +5755,7 @@ the corresponding statistics data.
The ``bucket_size`` field is used as a parameter for histogram statistics data.
It is only used by linear histogram statistics data, specifying the size of a
bucket.
bucket in the unit expressed by bits 4-11 of ``flags`` together with ``exponent``.
The ``name`` field is the name string of the statistics data. The name string
starts at the end of ``struct kvm_stats_desc``. The maximum length including