On Thu, Apr 22, 2010 at 11:47 AM, Milan Broz <mbroz@xxxxxxxxxx> wrote: > On 04/22/2010 08:08 PM, San Mehat wrote: >> On Thu, Apr 22, 2010 at 11:03 AM, Milan Broz <mbroz@xxxxxxxxxx> 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@xxxxxxxxxx) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
