Re: [boost] Ann: guid library

From: Lassi A. Tuura (lassi.tuura_at_[hidden])
Date: 2006-11-14 03:50:17

Hi,

> It seems from looking at the code that create_v4 will create  
> identically the same guid if two computers (or nodes in a parallel  
> computer) create a guid during the same second? This would be very  
> common in any parallel program. This is why the paper you cite  
> suggests using the MAC address of the computer for the lowest 6  
> octets.

In case it helps the discussion, over the last year or so we've had  
several reports of conflicting GUIDs generated independently in our  
applications.

As far as I know all cases involved multiprocessor systems generating  
GUIDs with MAC address.  We never managed to trap this live but our  
analysis indicates exactly the same piece of code executed exactly  
the same millisecond on several processors and at least two managed  
to generate the same 12 bits of randomness there are in GUIDs with  
MAC address.[*]

Our GUID library (from ext2fs) uses /dev/urandom to generate the  
random values, so it's pretty good; it's just that 12 bits of  
randomness is not a lot and a millisecond is a long time these days.

In light of this, I would recommend using fully random GUIDs by  
default.  Otherwise there is a very real probability of generating  
the same GUID several times.

Lassi

[*] Our best guess was that all the processes got synchronised  
through a system level lock such as accessing a file on NFS, which  
then acted as a barrier and released all processes to the GUID- 
generating code exactly the same time.