histograms: add script for parsing histogram data

#!/usr/bin/env python3

import sys

import json

import base64

import struct

buf = sys.stdin.readlines()

json = json.loads(" ".join(buf))

histogram = base64.b64decode(json["histogram"])

bucket_shift = json["bucket_shift"]

tsc_rate = json["tsc_rate"]

print("Latency histogram")

print("==============================================================================")

print("       Range in us     Cumulative    IO count")

so_far = 0

bucket = 0

total = 1

for i in range(0, 64 - bucket_shift):

    for j in range(0, (1 << bucket_shift)):

        index = (((i << bucket_shift) + j) * 8)

        total += int.from_bytes(histogram[index:index + 8], 'little')

for i in range(0, 64 - bucket_shift):

    for j in range(0, (1 << bucket_shift)):

        index = (((i << bucket_shift) + j)*8)

        count = int.from_bytes(histogram[index:index + 8], 'little')

        so_far += count

        last_bucket = bucket

        if i > 0:

            bucket = (1 << (i + bucket_shift - 1))

            bucket += ((j+1) << (i - 1))

        else:

            bucket = j+1

        start = last_bucket * 1000 * 1000 / tsc_rate

        end = bucket * 1000 * 1000 / tsc_rate

        so_far_pct = so_far * 100.0 / total

        if count > 0:

            print("%9.3f - %9.3f: %9.4f%%  (%9u)" % (start, end, so_far_pct, count))