Wireshark-dev: Re: [Wireshark-dev] Wireshark memory handling
From: "Anders Broman" <[email protected]>
Date: Thu, 15 Oct 2009 03:40:53 +0200
 

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Guy Harris
Sent: den 14 oktober 2009 21:32
To: Erlend Hamberg
Cc: [email protected]; [email protected]
Subject: Re: [Wireshark-dev] Wireshark memory handling


On Oct 13, 2009, at 11:00 AM, Erlend Hamberg wrote:

>> On Saturday 10. October 2009 03.48.29 Guy Harris wrote:
>>> The data Wireshark currently keeps in its address space that could 
>>> grow in size as the capture file grows are:
>>>
>>> 	the frame_data structure (epan/frame_data.h) - one structure 
>>> instance per packet;
>>
>> Ok, so - if my understanding is correct - for every packet that is 
>> read, an frame_data structure is created
>
>Yes.
>
>>> 	the text for some or all of the columns in all of the rows of
the 
>>> packet list (all, in current releases of Wireshark; some, in the 
>>> development branch);
>>
>> Ok, not much to save here after the introduction of the new packet 
>> list, I guess.
>
>There might be more we can save if we have efficient random access to
packets (even in 
>compressed files), as we can just re-dissect the packet whenever we
need the columns for 
>it.
>
>That could make sorting painful, however.
>
>>> The data from the frames in the capture file are not kept in 
>>> Wireshark's address space - they are read in as necessary, into a 
>>> small number of buffers (one for the main window, and one for each 
>>> packet window opened).  *HOWEVER*, if data from a frame is 
>>> reassembled into a higher-level multiple-frame packet, the result of

>>> the reassembly is, as noted, kept in Wireshark's address space.
>>
>> So, when Wireshark reads the capture file, if it finds a single-
frame 
>> packet, it will only create a frame_data structure in memory and 
>> possibly data from the dissector for that type of packet. But if the 
>> packet is made up of several frames, the packet is reassembled and 
>> kept in memory?
>
>Yes.
>
>> If so, do you think this could be changed?
>
>We probably need to keep the packet data in memory while it's being  
>reassembled and when it's dissected.
>
>Again, with efficient random access, we could free it when we're done  
>with it, and leave behind an array of frame numbers, starting offsets,

>and lengths, so that on the next reference the frames can be read, the

>data reassembled, and keep the data around, again, only while it's  
>needed.

I think this would be a huge improvment and well worth implementing.
Shall
we cut the gordian knot as suggested at Sharkfest and decompress
compressed files
to a temp file and read that to get around the compressed files problem
or drop the abillity to read compressed files and let the user
unkompress them before use.
Anders

>
>> Would it be worth it?
>
>Probably.  It would also mean that TShark would accumulate a lot less  
>memory, and perhaps be able to run much longer, when dissecting  
>packets (rather than just writing them to a file).
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