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Re: [Qemu-block] [Qemu-devel] [PATCH v2 1/2] block/accounting: introduce

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v2 1/2] block/accounting: introduce latency histogram
Date: Thu, 8 Mar 2018 21:58:10 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

08.03.2018 21:21, Vladimir Sementsov-Ogievskiy wrote:
08.03.2018 21:14, Vladimir Sementsov-Ogievskiy wrote:
08.03.2018 20:31, Eric Blake wrote:
On 03/06/2018 09:32 AM, Stefan Hajnoczi wrote:
On Wed, Feb 07, 2018 at 03:50:36PM +0300, Vladimir Sementsov-Ogievskiy wrote:
Introduce latency histogram statics for block devices.
For each accounted operation type latency region [0, +inf) is
divided into subregions by several points. Then, calculate
hits for each subregion.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>

According to Wikipedia and Mathworld, "intervals" and "bins" are
commonly used terms:

I suggest:

   typedef struct {
       /* The following histogram is represented like this:
        * 5|           *
        * 4|           *
        * 3| *         *
        * 2| *         *    *
        * 1| *    *    *    *
        *  +------------------
        *      10   50   100
        * BlockLatencyHistogram histogram = {
        *     .nbins = 4,
        *     .intervals = {10, 50, 100},
        *     .bins = {3, 1, 5, 2},
        * };

The name 'intervals' is still slightly ambiguous: does it hold the boundary point (0-10 for 10 slots, 10-50 for 40 slots, 50-100, for 50 slots, then 100-INF) or is it the interval size of each slot (first bin is 10 slots for 0-10, next bin is 50 slots wide so 10-60, next bin is 100 slots wide so 60-160, everything else is 160-INF).  But the ascii-art diagram plus the text is sufficient to resolve the intent if you keep that name (I don't have a suggestion for a better name).

Hm. these numbers are actually boundary points of histogram intervals, not intervals itself. And, wiki says "The bins are usually specified as consecutive, non-overlapping intervals of a variable.", so, intervals are bins.

So, what about:

1. interval_boundaries
2. bin_boundaries
3. boundaries

(and same names with s/_/-/ for qapi)

Also, now I doubt, is it a good idea to share same bin boundaries for each io type.

so, for qmp command, what about:

boundaries - optional, default boundaries for all io operations
boundaries-read - boundaries for read
boundaries-write - boundaries for write

so, call without any boundaries: drop _all_ histograms
call with only specific boundaries-*: set or reset _only_ corresponding specific histograms
call with only boundaries parameter: set or reset _all_ histograms
call with boundaries parameter and some of (or all, but it is not useful) specific boundaries-*: set or reset _all_ histograms, some to default  and some to specific.

Best regards,

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