Subject: Re: [PATCH RFC] pkt_sched: QFQ Plus: fair-queueing service at DRR cost

From: Paolo Valente <[email-masked]>
Date: Mon, 01 Oct 2012 17:50:58 +0200

Il 01/10/2012 17:31, Stephen Hemminger ha scritto:
> On Sun, 30 Sep 2012 19:40:49 +0200
> Paolo Valente <[email-masked]> wrote:
>
>> Hi,
>> this patch turns QFQ into QFQ+, a faster variant of QFQ that groups
>> classes into aggregates, and uses the original QFQ scheduling
>> algorithm to schedule aggregates instead of single classes. An
>> aggregate is made of at most M classes, all with the same weight and
>> maximum packet size.  M is equal to the minimum between tx_queue_len+1
>> and 8 (value chosen to get a good trade-off between execution time and
>> service guarantees). QFQ+ associates each aggregate with a budget
>> equal to the maximum packet size for the classes in the aggregate,
>> multiplied by the number of classes of the aggregate. Once selected an
>> aggregate for service, QFQ+ dequeues only the packets of its classes,
>> until the aggregate finishes its budget. Finally, within an aggregate,
>> classes are scheduled with DRR. In my tests, described below, the
>> execution time of QFQ+ with M=8 was from 16% to 31% lower than that of
>> QFQ, and close to that of DRR.
>>
>> QFQ+ does not use packet lengths for computing aggregate timestamps,
>> but budgets. Hence it does not need to modify any timestamp if the
>> head packet of a class changes. As a consequence, differently from
>> QFQ, which uses head-packet lengths to compute class timestamps, QFQ+
>> does not need further modifications to correctly schedule also
>> non-leaf classes and classes with non-FIFO qdiscs. Finally, QFQ+ is
>> more robust than QFQ against corruption of the data structures
>> implementing the bucket lists. A detailed description of QFQ+ can be
>> found in [1].
>>
>> As for service guarantees, thanks to the way how M is computed, the
>> service of QFQ+ is close to the one of QFQ. For example, as proved in
>> [1], under QFQ+ every packet of a given class is guaranteed the same
>> worst-case completion time as under QFQ, plus an additional delay
>> equal to the transmission time, at the rate reserved to the class, of
>> three maximum-size packet. See [1, Section 7.1] for a numerical
>> comparison among the packet delays guaranteed by QFQ+, QFQ and DRR.
>>
>> I measured the execution time of QFQ+, DRR and QFQ using the testing
>> environment [2]. In particular, for each scheduler I measured the
>> average total execution time of a packet enqueue plus a packet
>> dequeue.  For practical reasons, in this testing environment each
>> enqueue&dequeue is also charged for the cost of generating and
>> discarding an empty, fixed-size packet (using a free list). The
>> following table reports the results with an i7-2760QM, against four
>> different class sets. Time is measured in nanoseconds, while each set
>> or subset of classes is denoted as <num_classes>-w<weight>, where
>> <num_classes> and <weight> are, respectively, the number of classes
>> and the weight of every class in the set/subset (for example, 250-w1
>> stands for 250 classes with weight 1). For QFQ+, the table shows the
>> results for the two extremes for M: 1 and 8 (see [1, Section 7.2] for
>> results with other values of M and for more information).
>>
>>   -----------------------------------------------
>> | Set of  |      QFQ+ (M)     |   DRR      QFQ  |
>> | classes |    1          8   |                 |
>> |-----------------------------------------------|
>> | 1k-w1   |   89         63   |    56       81  |
>> |-----------------------------------------------|
>> | 500-w1, |                   |                 |
>> | 250-w2, |  102         71   |    87      103  |
>> | 250-w4  |                   |                 |
>> |-----------------------------------------------|
>> | 32k-w1  |  267        225   |   173      257  |
>> |-----------------------------------------------|
>> | 16k-w1, |                   |                 |
>> | 8k-w2,  |  253        187   |   252      257  |
>> | 8k-w4   |                   |                 |
>>   -----------------------------------------------
>>
>> About DRR, it achieves its best performance when all the classes have
>> the same weight. This is fortunate, because in such scenarios it is
>> actually pointless to use a fair-queueing scheduler, as the latter
>> would provide the same quality of service as DRR. In contrast, when
>> classes have differentiated weights and the better service properties
>> of QFQ+ make a difference, QFQ+ has better performance than DRR. It
>> happens mainly because QFQ+ dequeues packets in an order that causes
>> about 8% less cache misses than DRR. As for the number of
>> instructions, QFQ+ executes instead about 7% more instructions than
>> DRR, whereas QFQ executes from 25% to 34% more instructions than DRR.
>>
>> Paolo
>>
>> [1] P. Valente, "Reducing the Execution Time of Fair-Queueing Schedulers"
>> http://algo.ing.unimo.it/people/paolo/agg-sched/agg-sched.pdf
>>
>> [2] http://algo.ing.unimo.it/people/paolo/agg-sched/test-env.tgz
>>
>> Signed-off-by: Paolo Valente <[email-masked]>
> I like the improvement and the performance improvement.
> Is there some concern that changing the implementation this much might
> upset some people already using QFQ?
>
> What happens if an existing working QFQ config is used in QFQ+?
>
>
>
QFQ+ has the same interface as QFQ, so existing QFQ configs should work 
without problems (if i am not missing anything)