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

[Pino Toscano]

Hi,

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?

Thanks,
-- 
Pino Toscano
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 819 bytes
Desc: This is a digitally signed message part.
URL: <http://listman.redhat.com/archives/libguestfs/attachments/20160926/29b142d3/attachment.sig>