Re: [Xen-devel] blkback global resources

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.12 at 14:34, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> On Tue, 2012-03-27 at 11:22 +0100, Jan Beulich wrote:
>> >>> On 27.03.12 at 11:41, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
>> > On Mon, 2012-03-26 at 17:20 +0100, Ian Campbell wrote:
>> >> On Mon, 2012-03-26 at 16:56 +0100, Jan Beulich wrote:
>> >> > All the resources allocated based on xen_blkif_reqs are global in
>> >> > blkback. While (without having measured anything) I think that this
>> >> > is bad from a QoS perspective (not the least implied from a warning
>> >> > issued by Citrix'es multi-page-ring patches:
>> >> > 
>> >> >         if (blkif_reqs < BLK_RING_SIZE(order))
>> >> >                 printk(KERN_WARNING "WARNING: "
>> >> >                        "I/O request space (%d reqs) < ring order %ld, "
>> >> >                        "consider increasing %s.reqs to >= %ld.",
>> >> >                        blkif_reqs, order, KBUILD_MODNAME,
>> >> >                        roundup_pow_of_two(BLK_RING_SIZE(order)));
>> >> > 
>> >> > indicating that this _is_ a bottleneck), I'm otoh hesitant to convert
>> >> > this to per-instance allocations, as the amount of memory taken
>> >> > away from Dom0 for this may be not insignificant when there are
>> >> > many devices.
>> >> > 
>> > 
>> > What's your main concern on QoS? Lock contention? Starvation? Or any
>> > other things?
>> 
>> However you want to put it. Prior to the multi-page ring patches, we
>> have 64 pending requests (global) and 32 ring entries. Obviously,
>> bumping the ring size just to order 1 will already bring the number of
>> possible in-flight entries per device on par with those in-flight across
>> all devices. So _if_ someone really determined that a multi-page ring
>> helps performance, I wonder whether that was with manually
>> adjusted global pending request values (not said anywhere) or with
>> just a single frontend (not very close to real world scenarios).
>> 
> 
> Just to be precise, bumping order to 1 makes ring entries more than 64.

Iirc order 0 -> 32 entries, order 1 -> 64 entries.

Jan


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