Re: [PATCH] qapi: better docs for calc-dirty-rate and friends

[Markus Armbruster <armbru@redhat.com>]

Andrei Gudkov <gudkov.andrei@huawei.com> writes:

> Rewrote calc-dirty-rate documentation. Briefly described
> different modes of dirty page rate measurement. Added some
> examples. Fixed obvious grammar errors.
>
> Signed-off-by: Andrei Gudkov <gudkov.andrei@huawei.com>
> ---
>  qapi/migration.json | 107 +++++++++++++++++++++++++++++++-------------
>  1 file changed, 77 insertions(+), 30 deletions(-)
>
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 179af0c4d8..19b51444b5 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -1735,14 +1735,14 @@
>  ##
>  # @DirtyRateStatus:
>  #
> -# An enumeration of dirtyrate status.
> +# Dirty page rate measurement status.
>  #
> -# @unstarted: the dirtyrate thread has not been started.
> +# @unstarted: measuring thread has not been started yet
>  #
> -# @measuring: the dirtyrate thread is measuring.
> +# @measuring: measuring thread is running
>  #
> -# @measured: the dirtyrate thread has measured and results are
> -#     available.
> +# @measured: dirty page rate is measured and the results are
> +#     available
>  #
>  # Since: 5.2
>  ##
> @@ -1752,13 +1752,14 @@
>  ##
>  # @DirtyRateMeasureMode:
>  #
> -# An enumeration of mode of measuring dirtyrate.
> +# Method used to measure dirty page rate.  Differences between
> +# available methods are explained in @calc-dirty-rate.
>  #
> -# @page-sampling: calculate dirtyrate by sampling pages.
> +# @page-sampling: use page sampling
>  #
> -# @dirty-ring: calculate dirtyrate by dirty ring.
> +# @dirty-ring: use dirty ring
>  #
> -# @dirty-bitmap: calculate dirtyrate by dirty bitmap.
> +# @dirty-bitmap: use dirty bitmap
>  #
>  # Since: 6.2
>  ##
> @@ -1768,26 +1769,25 @@
>  ##
>  # @DirtyRateInfo:
>  #
> -# Information about current dirty page rate of vm.
> +# Information about measured dirty page rate.
>  #
>  # @dirty-rate: an estimate of the dirty page rate of the VM in units
> -#     of MB/s, present only when estimating the rate has completed.
> +#     of MiB/s. Value is present only when @status is 'measured'.

For consistency, please put two spaces between setences.

>  #
> -# @status: status containing dirtyrate query status includes
> -#     'unstarted' or 'measuring' or 'measured'
> +# @status: current status of dirty page rate measurements
>  #
>  # @start-time: start time in units of second for calculation
>  #
> -# @calc-time: time in units of second for sample dirty pages
> +# @calc-time: time period in units of second for which dirty page
> +#     rate was measured

Maybe

   # @calc-time: time period for which dirty page rate was measured
   #     (in seconds)

>  #
> -# @sample-pages: page count per GB for sample dirty pages the default
> -#     value is 512 (since 6.1)
> +# @sample-pages: number of sampled pages per each GiB of guest

per GiB

> +#     memory.  Value is valid only in page-sampling mode (Since 6.1)

Suggest "Valid only in ..."

>  #
> -# @mode: mode containing method of calculate dirtyrate includes
> -#     'page-sampling' and 'dirty-ring' (Since 6.2)
> +# @mode: mode that was used to measure dirty page rate (Since 6.2)
>  #
> -# @vcpu-dirty-rate: dirtyrate for each vcpu if dirty-ring mode
> -#     specified (Since 6.2)
> +# @vcpu-dirty-rate: dirty rate for each vCPU if dirty-ring mode
> +#     was specified (Since 6.2)
>  #
>  # Since: 5.2
>  ##
> @@ -1803,15 +1803,50 @@
>  ##
>  # @calc-dirty-rate:
>  #
> -# start calculating dirty page rate for vm
> -#
> -# @calc-time: time in units of second for sample dirty pages
> -#
> -# @sample-pages: page count per GB for sample dirty pages the default
> -#     value is 512 (since 6.1)
> -#
> -# @mode: mechanism of calculating dirtyrate includes 'page-sampling'
> -#     and 'dirty-ring' (Since 6.1)
> +# Starts measuring dirty page rate of the VM.  Results can be

Imperative mood: "Start measuring ..."

> +# retrieved with @query-dirty-rate after measurements are completed.
> +#
> +# Dirty page rate is the number of pages changed in a given time
> +# period expressed in MiB/s.  The following methods of calculation
> +# are available:
> +#
> +# 1. In page sampling mode, a random subset of pages are selected
> +#    and hashed twice: once in the beginning of measurement time

Suggest "once at the beginning"

> +#    period, another one -- in the end.  If two hashes for some page

Suggest ", and once again at the end".

> +#    are different, the page is counted as changed.  Since this
> +#    method relies on sampling and hashing, calculated dirty page
> +#    rate is only the estimation of its true value.  Setting
> +#    @sample-pages to higher value improves estimation quality but

Suggest "Increasing @sample-pages improves estimation quality at the
cost ..."

> +#    at the cost of higher computational overhead.
> +#
> +# 2. Dirty bitmap mode captures writes to memory by temporarily
> +#    revoking write access to all pages and counting page faults.

Comma before "and".

> +#    Information about modified pages is collected into bitmap,

"into a bitmap"

> +#    where each bit corresponds to one guest page.  This mode
> +#    requires that KVM accelerator property "dirty-ring-size=N"

Suggest just "dirty-ring-size" (omit "=N").

> +#    is *not* set.
> +#
> +# 3. Dirty ring mode is similar to dirty bitmap mode, but the
> +#    information about modified pages is collected into ring buffer.
> +#    This mode tracks page modification per each vCPU separately.

Either "for each vCPU" or "per vCPU".

> +#    It requires that KVM accelerator property "dirty-ring-size=N"
> +#    is set.

Suggest just "dirty-ring-size" (omit "=N").

> +#
> +# @calc-time: time period in units of second for which dirty page rate
> +#    is calculated.  Note that larger @calc-time values will typically
> +#    result in smaller dirty page rates because page dirtying is a
> +#    one-time event.  Once some page is counted as dirty during
> +#    @calc-time period, further writes to this page will not increase
> +#    dirty page rate anymore.

Please indent one more, for consistency.

> +#
> +# @sample-pages: number of sampled pages per each GiB of guest memory.
> +#     Default value is 512.  For 4KiB guest pages this corresponds to
> +#     sampling ratio of 0.2%.  This argument is used only in page
> +#     sampling mode. (Since 6.1)

Two spaces between '.' and '(', please.

> +#
> +# @mode: mechanism for tracking dirty pages.  Default value is
> +#    'page-sampling'.  Others are 'dirty-bitmap' and 'dirty-ring'.
> +#    (Since 6.1)
>  #
>  # Since: 5.2
>  #
> @@ -1828,9 +1863,21 @@
>  ##
>  # @query-dirty-rate:
>  #
> -# query dirty page rate in units of MB/s for vm
> +# Query results of the most recent invocation of @calc-dirty-rate.
>  #
>  # Since: 5.2
> +#
> +# Examples:
> +#
> +# 1. Measurement is in progress:
> +#
> +# <- {"status": "measuring", "sample-pages": 512,
> +#     "mode": "page-sampling", "start-time": 3665220, "calc-time": 10}
> +#
> +# 2. Measurement has been completed:
> +#
> +# <- {"status": "measured", "sample-pages": 512, "dirty-rate": 108,
> +#     "mode": "page-sampling", "start-time": 3665220, "calc-time": 10}
>  ##
>  { 'command': 'query-dirty-rate', 'returns': 'DirtyRateInfo' }

This is *sooo* much better than before.  Thank you!

An R-by from a migration maintainer would be nice.

If you agree with my suggestions, I can apply them in my tree, saving
you a respin.  Let me know.

Acked-by: Markus Armbruster <armbru@redhat.com>