Re: [Xen-devel] [PATCH 04/10] xen/blkfront: separate ring information to an new struct

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

El 19/02/15 a les 3.05, Bob Liu ha escrit:
> 
> 
> On 02/19/2015 02:08 AM, Felipe Franciosi wrote:
>>> -----Original Message-----
>>> From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx]
>>> Sent: 18 February 2015 17:38
>>> To: Roger Pau Monne
>>> Cc: Bob Liu; xen-devel@xxxxxxxxxxxxx; David Vrabel; linux-
>>> kernel@xxxxxxxxxxxxxxx; Felipe Franciosi; axboe@xxxxxx; hch@xxxxxxxxxxxxx;
>>> avanzini.arianna@xxxxxxxxx
>>> Subject: Re: [PATCH 04/10] xen/blkfront: separate ring information to an new
>>> struct
>>>
>>> On Wed, Feb 18, 2015 at 06:28:49PM +0100, Roger Pau Monné wrote:
>>>> El 15/02/15 a les 9.18, Bob Liu ha escrit:
>>>> AFAICT you seem to have a list of persistent grants, indirect pages
>>>> and a grant table callback for each ring, isn't this supposed to be
>>>> shared between all rings?
>>>>
>>>> I don't think we should be going down that route, or else we can hoard
>>>> a large amount of memory and grants.
>>>
>>> It does remove the lock that would have to be accessed by each ring thread 
>>> to
>>> access those. Those values (grants) can be limited to be a smaller value 
>>> such
>>> that the overall number is the same as it was with the previous version. As 
>>> in:
>>> each ring has = MAX_GRANTS / nr_online_cpus().
>>>>
>>
>> We should definitely be concerned with the amount of memory consumed on the 
>> backend for each plugged virtual disk. We have faced several problems in 
>> XenServer around this area before; it drastically affects VBD scalability 
>> per host.
>>
> 
> Right, so we have to keep both the lock and the amount of memory
> consumed in mind.
> 
>> This makes me think that all the persistent grants work was done as a 
>> workaround while we were facing several performance problems around 
>> concurrent grant un/mapping operations. Given all the recent submissions 
>> made around this (grant ops) area, is this something we should perhaps 
>> revisit and discuss whether we want to continue offering persistent grants 
>> as a feature?
>>
> 
> Agree, Life would be easier if we can remove the persistent feature.

I was thinking about this yesterday, and IMHO I think we should remove
persistent grants now while it's not too entangled, leaving it for later
will just make our life more miserable.

While it's true that persistent grants provide a throughput increase by
preventing grant table operations and TLB flushes, it has several
problems that cannot by avoided:

 - Memory/grants hoarding, we need to reserve the same amount of memory
as the amount of data that we want to have in-flight. While this is not
so critical for memory, it is for grants, since using too many grants
can basically deadlock other PV interfaces. There's no way to avoid this
since it's the design behind persistent grants.

 - Memcopy: guest needs to perform a memcopy of all data that goes
through blkfront. While not so critical, Felipe found systems were
memcopy was more expensive than grant map/unmap in the backend (IIRC
those were AMD systems).

 - Complexity/interactions: when persistent grants was designed number
of requests was limited to 32 and each request could only contain 11
pages. This means we had to use 352 pages/grants which was fine. Now
that we have indirect IO and multiqueue in the horizon this number has
gone up by orders of magnitude, I don't think this is viable/useful any
more.

If Konrad/Bob agree I would like to send a patch to remove persistent
grants and then have the multiqueue series rebased on top of that.

Roger.


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