Re: low pending handshake limit

["Moritz Wanzenböck" <moritz.wanzenboeck@linbit.com>]

Hi Chuck,

On Mo, Sep 4 2023 at 15:13:25 +0000, Chuck Lever III 
<chuck.lever@oracle.com> wrote:
> Hi-
> 
>>  On Sep 4, 2023, at 8:39 AM, Moritz Wanzenböck 
>> <moritz.wanzenboeck@linbit.com> wrote:
>> 
>>  Hi all,
>> 
>>  I'm currently working on enabling TLS support for DRBD, so I'm very 
>> keen to use the handshake infrastructure.
> 
> I'm happy to see the handshake infrastructure get more usage.
> 
> 
>>  During testing I noticed that the allowed number of pending 
>> handshakes is quite low. This seems to stem from the following 
>> calculation:
>> 
>>  /*
>>  * Arbitrary limit to prevent handshakes that do not make
>>  * progress from clogging up the system. The cap scales up
>>  * with the amount of physical memory on the system.
>>  */
>>  si_meminfo(&si);
>>  tmp = si.totalram / (25 * si.mem_unit);
>>  hn->hn_pending_max = clamp(tmp, 3UL, 50UL);
>> 
>>  Which, for the typical VMs I use for testing (1Gi RAM), ends up 
>> being just 3 handshakes. The limits in general seem too low also in 
>> the best case. If a node just booted, and would start connecting to 
>> all configured DRBD devices, we could easily hit even the upper 
>> limit of 50.
>> 
>>  Also the calculation used doesn't seem to make too much sense to 
>> me. It allows more handshakes when using a smaller page size?
>> 
>>  Would it be possible to increase the number of pending handshakes?
> 
> IIRC I added the dynamic computation in response to a review
> comment from Paolo (cc'd). I think the limit values are arbitrary,
> we just want a sensible cap on the number of pending handshakes,
> and on smaller systems, that limit should be a smaller value.
> 
> It's true that a handshake can fail if that limit is hit, but
> the consumer ought to be able to retry after a brief delay in
> that case.
> 
> I am open to discussing changes if retrying proves to be a
> challenge.

Thanks for the explanation. Actually, retrying is not an issue. I was 
initially
vary, I thought the requests remained pending until the handshake was 
complete.
Looks like I was wrong about that, it's just pending until the netlink 
message
is sent to the user space utility.

Best regards,
Moritz