Re: [PATCH] md: dm-crypt: Add option to re-use a new global work-queue.

[San Mehat <san@google.com>]

On Thu, Apr 22, 2010 at 11:47 AM, Milan Broz <mbroz@redhat.com> wrote:
> On 04/22/2010 08:08 PM, San Mehat wrote:
>> On Thu, Apr 22, 2010 at 11:03 AM, Milan Broz <mbroz@redhat.com> wrote:
>>> On 04/22/2010 07:48 PM, San Mehat wrote:
>>>> Typically, dm-crypt instances each have their own set of kcrypt/kcrypt_io
>>>> work-queues. This patch adds an option which will create one set of
>>>> work-queues on init, and re-uses them for all dm-crypt target instances.
>
>>> Can you explain the real reason for this patch?
>>>
>>
>> Sure, I'd be happy to explain.
>
> (Please add this always to patch header.)
>

Will do - thanks.

>>
>>   Upcoming versions of android are about to start using dm/dm-crypt
>> heavily, having
>> a large number of small dm-crypt instances running on the device (hard
>> to tell just
>> how many, but i've seen cases where up to 50 or 60 instances may be
>> running). This ends up creating 100 - 120 kernel threads, and I was
>> simply trying to cut that down.
>
> Sorry, but I don't take this argument. "Too many notes!" :-)
>
> So the problem is with memory allocation? Scheduler? Or where?
> Kernel threads should be cheap.
>

Well the initial consideration was towards memory overhead with so
many threads that don't do much (in our use-case) on an embedded
device.

> If you need 60 crypt devices, you almost surely hit at least starvation
> problem with one global queue!
> (Just curious - what are these crypt devices doing?)

The crypt devices are providing small read-only encrypted file-systems
- whose backing files exist on an external FAT file-system, and are
created on-demand as needed. In this usage scenario, we'll only see
typically a few of these devices being simultaneously accessed, (and
the sd-card throughput is definitely the long-pole in the performance
profile, so even when I beat on 80 or 90 concurrent instances, we're
mainly waiting for mmcqd to complete transactions).

>
>> I'd be more than happy to discuss alternatives; but do we *really*
>> need 2 work-queue threads per instance?
>
> yes.

What if we made a note in the Kconfig advising against using the option in
stacked configurations? (Or even make it depend on CONFIG_EMBEDDED)

Thanks for your time,

-san

>
> For separate io queue - see commit cabf08e4d3d1181d7c408edae97fb4d1c31518af
>
> | Add post-processing queue (per crypt device) for read operations.
>
> | Current implementation uses only one queue for all operations
> | and this can lead to starvation caused by many requests waiting
> | for memory allocation. But the needed memory-releasing operation
> | is queued after these requests (in the same queue).
>
>
> (and there were another problem with async crypt - callback is called
> in interrupt context, bio must be submitted from separate workqueue IIRC)
>
>>> (cc: Alasdair - I think he will not accept the patch anyway.)
>>
>> Probably not, but at least we can get the discussion going :)
>
> I am not saying that I do not want to discuss this - but we must know
> the real problems many queues are causing first.
> And then think about possible solutions.
>
> Milan
>



-- 
San Mehat  |  Staff Software Engineer  |  Android  |  Google Inc.
415.366.6172 (san@google.com)