Re: [ANNOUNCE] xen ocaml tools

[Patrick Colp <pjcolp@cs.ubc.ca>]

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Vincent Hanquez wrote:
> Patrick Colp wrote:
>> I'm really excited to see somebody else working on an OCaml XenStore! 
>> I was wondering if you could tell me what the difference are between 
>> this implementation and the one I recently released to the community?
>>   
> this is a bit hard to tell without testing your version.
> 
> but i think the main difference is the way we handle transactions, which 
> should provide a stable average time to commit transactions when having 
> lots of xenstore traffic from guests.

I think you're thinking of my initial release last year. The version I released 
a few months ago also has an in-memory store and greatly improved transactions. 
It was motivated by the need to survive things like DoS attacks.

I wrote a little attack program (in OCaml) which runs from any DomU and brought 
the original xenstored to its knees. With the attack going, it's impossible to 
bring a new domain up -- it just hangs forever attempting to bring it up. 
Basically, the attack just hammers xenstored with micro-transactions. With the 
original transaction system, which allows the first committing transaction in a 
generation to win, long transactions could never complete. I implemented 
transactions that would enable all concurrent but non-conflicting transactions 
to commit. This made my version of xenstored resilient to the attack.

I played around with this with your version too, but found that, while it would 
not hang forever while attempting to load a domain, it would instead die after a 
few seconds with the following error:

Error: (2, 'No such file or directory')

I tried with with the eagain mode thing (random dropping of 1/3 of all 
transactions) both enabled and disabled, but it had the same effect (except that 
with the mode enabled, 1/3 of all transactions would fail regardless of if they 
should or not).

I've been reading over your code and noticed that you seem to have a 
mini-implementation of libxc. I was wondering why you chose to do this over 
using the pre-existing libxenctrl? Does this make the final executable smaller?


Patrick

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