From 942d9e89524c135615e557fffa144104ea8fb361 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Thu, 14 Jul 2022 07:29:57 -0400 Subject: Documentation: kvm: clarify histogram units 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 --- Documentation/virt/kvm/api.rst | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) (limited to 'Documentation') diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 48bf6e49a7de..6e090fb96a0e 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -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 -- cgit v1.2.3