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+=================
+ Perf histograms
+=================
+
+The perf histograms build on perf counters infrastructure. Histograms are built for a number of counters and simplify gathering data on which groups of counter values occur most often over time. 
+Perf histograms are currently unsigned 64-bit integer counters, so they're mostly useful for time and sizes. Data dumped by perf histogram can then be feed into other analysis tools/scripts.
+
+Access
+------
+
+The perf histogram data are accessed via the admin socket.  For example::
+
+   ceph daemon osd.0 perf histogram schema
+   ceph daemon osd.0 perf histogram dump
+
+
+Collections
+-----------
+
+The histograms are grouped into named collections, normally representing a subsystem or an instance of a subsystem.  For example, the internal ``throttle`` mechanism reports statistics on how it is throttling, and each instance is named something like::
+
+
+    op_r_latency_out_bytes_histogram
+    op_rw_latency_in_bytes_histogram
+    op_rw_latency_out_bytes_histogram
+    ...
+
+
+Schema
+------
+
+The ``perf histogram schema`` command dumps a json description of which values are available, and what their type is.  Each named value as a ``type`` bitfield, with the 5-th bit always set and following bits defined.
+
++------+-------------------------------------+
+| bit  | meaning                             |
++======+=====================================+
+| 1    | floating point value                |
++------+-------------------------------------+
+| 2    | unsigned 64-bit integer value       |
++------+-------------------------------------+
+| 4    | average (sum + count pair)          |
++------+-------------------------------------+
+| 8    | counter (vs gauge)                  |
++------+-------------------------------------+
+
+In other words, histogram of type "18" is a histogram of unsigned 64-bit integer values (16 + 2).
+
+Here is an example of the schema output::
+
+    {
+        "AsyncMessenger::Worker-0": {},
+        "AsyncMessenger::Worker-1": {},
+        "AsyncMessenger::Worker-2": {},
+        "mutex-WBThrottle::lock": {},
+        "objecter": {},
+        "osd": {
+            "op_r_latency_out_bytes_histogram": {
+                "type": 18,
+                "description": "Histogram of operation latency (including queue time) +   da  ta read",
+                "nick": ""
+            },
+            "op_w_latency_in_bytes_histogram": {
+                "type": 18,
+                "description": "Histogram of operation latency (including queue time) +   da  ta written",
+                "nick": ""
+            },
+            "op_rw_latency_in_bytes_histogram": {
+                "type": 18,
+                "description": "Histogram of rw operation latency (including queue time)   +   data written",
+                "nick": ""
+            },
+            "op_rw_latency_out_bytes_histogram": {
+                "type": 18,
+                "description": "Histogram of rw operation latency (including queue time)   +   data read",
+                "nick": ""
+            }
+        }
+    }
+
+
+Dump
+----
+
+The actual dump is similar to the schema, except that there are actual value groups.  For example::
+
+    "osd": {
+        "op_r_latency_out_bytes_histogram": {
+            "axes": [
+                {
+                    "name": "Latency (usec)",
+                    "min": 0,
+                    "quant_size": 100000,
+                    "buckets": 32,
+                    "scale_type": "log2",
+                    "ranges": [
+                        {
+                            "max": -1
+                        },
+                        {
+                            "min": 0,
+                            "max": 99999
+                        },
+                        {
+                            "min": 100000,
+                            "max": 199999
+                        },
+                        {
+                            "min": 200000,
+                            "max": 399999
+                        },
+                        {
+                            "min": 400000,
+                            "max": 799999
+                        },
+                        {
+                            "min": 800000,
+                            "max": 1599999
+                        },
+                        {
+                            "min": 1600000,
+                            "max": 3199999
+                        },
+                        {
+                            "min": 3200000,
+                            "max": 6399999
+                        },
+                        {
+                            "min": 6400000,
+                            "max": 12799999
+                        },
+                        {
+                            "min": 12800000,
+                            "max": 25599999
+                        },
+                        {
+                            "min": 25600000,
+                            "max": 51199999
+                        },
+                        {
+                            "min": 51200000,
+                            "max": 102399999
+                        },
+                        {
+                            "min": 102400000,
+                            "max": 204799999
+                        },
+                        {
+                            "min": 204800000,
+                            "max": 409599999
+                        },
+                        {
+                            "min": 409600000,
+                            "max": 819199999
+                        },
+                        {
+                            "min": 819200000,
+                            "max": 1638399999
+                        },
+                        {
+                            "min": 1638400000,
+                            "max": 3276799999
+                        },
+                        {
+                            "min": 3276800000,
+                            "max": 6553599999
+                        },
+                        {
+                            "min": 6553600000,
+                            "max": 13107199999
+                        },
+                        {
+                            "min": 13107200000,
+                            "max": 26214399999
+                        },
+                        {
+                            "min": 26214400000,
+                            "max": 52428799999
+                        },
+                        {
+                            "min": 52428800000,
+                            "max": 104857599999
+                        },
+                        {
+                            "min": 104857600000,
+                            "max": 209715199999
+                        },
+                        {
+                            "min": 209715200000,
+                            "max": 419430399999
+                        },
+                        {
+                            "min": 419430400000,
+                            "max": 838860799999
+                        },
+                        {
+                            "min": 838860800000,
+                            "max": 1677721599999
+                        },
+                        {
+                            "min": 1677721600000,
+                            "max": 3355443199999
+                        },
+                        {
+                            "min": 3355443200000,
+                            "max": 6710886399999
+                        },
+                        {
+                            "min": 6710886400000,
+                            "max": 13421772799999
+                        },
+                        {
+                            "min": 13421772800000,
+                            "max": 26843545599999
+                        },
+                        {
+                            "min": 26843545600000,
+                            "max": 53687091199999
+                        },
+                        },
+                        {
+                            "min": 53687091200000
+                        }
+                    ]
+                },
+                {
+                    "name": "Request size (bytes)",
+                    "min": 0,
+                    "quant_size": 512,
+                    "buckets": 32,
+                    "scale_type": "log2",
+                    "ranges": [
+                        {
+                            "max": -1
+                        },
+                        {
+                            "min": 0,
+                            "max": 511
+                        },
+                        {
+                            "min": 512,
+                            "max": 1023
+                        },
+                        {
+                            "min": 1024,
+                            "max": 2047
+                        },
+                        {
+                            "min": 2048,
+                            "max": 4095
+                        },
+                        {
+                            "min": 4096,
+                            "max": 8191
+                        },
+                        {
+                            "min": 8192,
+                            "max": 16383
+                        },
+                        {
+                            "min": 16384,
+                            "max": 32767
+                        },
+                        {
+                            "min": 32768,
+                            "max": 65535
+                        },
+                        {
+                            "min": 65536,
+                            "max": 131071
+                        },
+                        {
+                            "min": 131072,
+                            "max": 262143
+                        },
+                        {
+                            "min": 262144,
+                            "max": 524287
+                        },
+                        {
+                            "min": 524288,
+                            "max": 1048575
+                        },
+                        {
+                            "min": 1048576,
+                            "max": 2097151
+                        },
+                        {
+                            "min": 2097152,
+                            "max": 4194303
+                        },
+                        {
+                            "min": 4194304,
+                            "max": 8388607
+                        },
+                        {
+                            "min": 8388608,
+                            "max": 16777215
+                        },
+                        {
+                            "min": 16777216,
+                            "max": 33554431
+                        },
+                        {
+                            "min": 33554432,
+                            "max": 67108863
+                        },
+                        {
+                            "min": 67108864,
+                            "max": 134217727
+                        },
+                        {
+                            "min": 134217728,
+                            "max": 268435455
+                        },
+                        {
+                            "min": 268435456,
+                            "max": 536870911
+                        },
+                        {
+                            "min": 536870912,
+                            "max": 1073741823
+                        },
+                        {
+                            "min": 1073741824,
+                            "max": 2147483647
+                        },
+                        {
+                            "min": 2147483648,
+                            "max": 4294967295
+                        },
+                        {
+                            "min": 4294967296,
+                            "max": 8589934591
+                        },
+                        {
+                            "min": 8589934592,
+                            "max": 17179869183
+                        },
+                        {
+                            "min": 17179869184,
+                            "max": 34359738367
+                        },
+                        {
+                            "min": 34359738368,
+                            "max": 68719476735
+                        },
+                        {
+                            "min": 68719476736,
+                            "max": 137438953471
+                        },
+                        {
+                            "min": 137438953472,
+                            "max": 274877906943
+                        },
+                        {
+                            "min": 274877906944
+                        }
+                    ]
+                }
+            ],
+            "values": [
+                [
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
+                    0,
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+                ],
+                [
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+                    0,
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+                ],
+                [
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+                ],
+                [
+                    0,
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+                ],
+                [
+                    0,
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+                ],
+                [
+                    0,
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+                ],
+                [
+                    0,
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+                ],
+                [
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+                ],
+                [
+                    0,
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+                    0,
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+                ]
+            ]
+        }
+    },
+
+This represents the 2d histogram, consisting of 9 history entrires and 32 value groups per each history entry. 
+"Ranges" element denote value bounds for each of value groups. "Buckets" denote amount of value groups ("buckets"), 
+"Min" is a minimum accepted valaue, "quant_size" is quantization unit and "scale_type" is either "log2" (logarhitmic
+scale) or "linear" (linear scale).
+You can use histogram_dump.py tool (see src/tools/histogram_dump.py) for quick visualisation of existing histogram
+data.