Question

i perform performance tests for a few java applications. Applications produce very big log files( it can be 7-10 GB) during the test . I need to trim these log files between specific dates and time. currently, i use python script, which parses log timestamps in datetime python object and print only matched strings. But this solution is very slow. 5 GB log is parsed about 25 minutes
Obviously entries in log file is sequentially and i don’t need to read all file line by line.
I thought about reading file from the start and from the end, until condition is matched and print files between matched number of lines. But i don’t know how can i read file from the backwards, without downloading it to the memory.

Please, can you suggest me any suitibale solution for this case.

here is part of python script:

      lfmt = '%Y-%m-%d %H:%M:%S'
      file = open(filename, 'rU')
      normal_line = ''
      for line in file:
        if line[0] == '[':
          ltimestamp = datetime.strptime(line[1:20], lfmt)

          if ltimestamp >= str and ltimestamp <= end:
            normal_line = 'True'
        else:
          normal_line = ''

      if normal_line:
        print line,

Answer 1

As the data is sequential if the start and end of the region of interest are near the beginning of the file, then reading from the end of the file (to find the matching end point) is still a bad solution!

I’ve written some code that will quickly find the start and end points as you require, this approach is called binary search and is similar to the clasic childrens “higher or lower” guessing game!

The script reads a trial line mid-way between lower_bounds and upper_bounds (initially the SOF and EOF), and checks the match criteria. If the sought line is earlier, then it guesses again by reading a line half-way between the lower_bound and the previous read trial (if its higher then it splits between its guess and the upper bound). So you keep iterating between upper and lower bounds – this yields the fastest possible “on average” solution.

This should be a real quick solution (log to the base 2 of the number of lines!!). For example in the worst possible case (finding line 999 out of a 1000 lines), using binary search would take just 9 line reads! (from a billion lines would take just 30…)

Assumptions for the below code:

• Every line starts with time information.
• The times are unique – If not, when a match is found you’ll have to check backwards or forwards to include or exclude all entries with matching time, as appropriate (if required).
• Amusingly this is a recursive function, so the number of lines of your file is limited to 2**1000 (luckily this allows for quite a large file…)

Further:

• This could be adapted to read in arbitrary blocks, rather than by line, if preferred. As suggested by J.F. Sebastian.
• In my original answerI suggested this approach but using linecache.getline, while this is possible its inappropriate for large files as it reads the whole file into memory (thus file.seek() is superior), thanks to TerryE and J.F. Sebastian for pointing that out.

import datetime

def match(line):
    lfmt = '%Y-%m-%d %H:%M:%S'
    if line[0] == '[':
        return datetime.datetime.strptime(line[1:20], lfmt)

def retrieve_test_line(position):
    file.seek(position,0)
    file.readline()  # avoids reading partial line, which will mess up match attempt
    new_position = file.tell() # gets start of line position
    return file.readline(), new_position

def check_lower_bound(position):
    file.seek(position,0)
    new_position = file.tell() # gets start of line position
    return file.readline(), new_position

def find_line(target, lower_bound, upper_bound):
    trial = int((lower_bound + upper_bound) /2)
    inspection_text, position = retrieve_test_line(trial)
    if position == upper_bound:
        text, position = check_lower_bound(lower_bound)
        if match(text) == target:
            return position
        return # no match for target within range
    matched_position = match(inspection_text)
    if matched_position == target:
        return position
    elif matched_position < target:
        return find_line(target, position, upper_bound)
    elif matched_position > target:
        return find_line(target, lower_bound, position)
    else:
        return # no match for target within range

lfmt = '%Y-%m-%d %H:%M:%S'
# start_target =  # first line you are trying to find:
start_target =  datetime.datetime.strptime("2012-02-01 13:10:00", lfmt)
# end_target =  # last line you are trying to find:
end_target =  datetime.datetime.strptime("2012-02-01 13:39:00", lfmt)
file = open("log_file.txt","r")
lower_bound = 0
file.seek(0,2) # find upper bound
upper_bound = file.tell()

sequence_start = find_line(start_target, lower_bound, upper_bound)

if sequence_start or sequence_start == 0: #allow for starting at zero - corner case
    sequence_end = find_line(end_target, sequence_start, upper_bound)
    if not sequence_end:
        print "start_target match: ", sequence_start
        print "end match is not present in the current file"
else:
    print "start match is not present in the current file"

if (sequence_start or sequence_start == 0) and sequence_end:
    print "start_target match: ", sequence_start
    print "end_target match: ", sequence_end
    print
    print start_target, 'target'
    file.seek(sequence_start,0)
    print file.readline()
    print end_target, 'target'
    file.seek(sequence_end,0)
    print file.readline()