[Libguestfs] [PATCH] Improve memory management of nbdkit python plugin example

[Carl-Daniel Hailfinger]

Hi,

sorry for the resend. My mailer mangled the previous attempt.

On 26.09.2016 17:19, Pino Toscano wrote:
> On Monday, 26 September 2016 17:07:41 CEST Carl-Daniel Hailfinger wrote:
>> the nbdkit python plugin example has suboptimal memory management:
>> - it creates the disk image as a string on init
>> - it casts the string to bytearray on every read
>> - it copies the string before and the string after the written region,
>> then reassembles those pieces together with the written region to a new
>> disk image string
>>
>> This is not a problem as long as the image is small, but in my tests
>> with a 5 GB sized image nbdkit already used 15 GB RAM directly after
>> startup, and even more (20-25 GB) on the first write.
>>
>> This changes the code to use bytearray everywhere and use the proper
>> methods to change bytearray objects directly. With the patch applied,
>> nbdkit with a 5 GB image will still only use 5 GB RAM even during heavy
>> read/write activity.
>>
>> Regards,
>> Carl-Daniel
>>
>> diff -r 521366d1854b -r d7d5078d08c7 plugins/python/example.py
>> --- a/plugins/python/example.py	Sun Jul 10 17:10:30 2016 +0100
>> +++ b/plugins/python/example.py	Sun Sep 25 05:04:02 2016 +0200
>> @@ -29,7 +29,7 @@
>>  # reconnect to the same server you should see the same disk.  You
>>  # could also put this into the handle, so there would be a fresh disk
>>  # per handle.
>> -disk = "\0" * (1024*1024);
>> +disk = bytearray(1024 * 1024)
>>  
>>  # This just prints the extra command line parameters, but real plugins
>>  # should parse them and reject any unknown parameters.
>> @@ -50,9 +50,9 @@
>>  
>>  def pread(h, count, offset):
>>      global disk
>> -    return bytearray (disk[offset:offset+count])
>> +    return disk[offset:offset+count]
>>  
>>  def pwrite(h, buf, offset):
>>      global disk
>>      end = offset + len (buf)
>> -    disk = disk[:offset] + buf + disk[end:]
>> +    disk[offset:end] = buf
> I'd look fine to me -- I guess it should work with both Python 2 and 3,
> right?

Indeed. By the way, does anyone know if there is a Python 3 plugin
infrastructure for nbdkit?

Regards,
Carl-Daniel