Re: [PATCH V3 00/11] block-throttle: add .high limit

[Paolo Valente]

> Il giorno 05 ott 2016, alle ore 20:30, Shaohua Li <shli@xxxxxx> ha scritto:
> 
> On Wed, Oct 05, 2016 at 10:49:46AM -0400, Tejun Heo wrote:
>> Hello, Paolo.
>> 
>> On Wed, Oct 05, 2016 at 02:37:00PM +0200, Paolo Valente wrote:
>>> In this respect, for your generic, unpredictable scenario to make
>>> sense, there must exist at least one real system that meets the
>>> requirements of such a scenario.  Or, if such a real system does not
>>> yet exist, it must be possible to emulate it.  If it is impossible to
>>> achieve this last goal either, then I miss the usefulness
>>> of looking for solutions for such a scenario.
>>> 
>>> That said, let's define the instance(s) of the scenario that you find
>>> most representative, and let's test BFQ on it/them.  Numbers will give
>>> us the answers.  For example, what about all or part of the following
>>> groups:
>>> . one cyclically doing random I/O for some second and then sequential I/O
>>> for the next seconds
>>> . one doing, say, quasi-sequential I/O in ON/OFF cycles
>>> . one starting an application cyclically
>>> . one playing back or streaming a movie
>>> 
>>> For each group, we could then measure the time needed to complete each
>>> phase of I/O in each cycle, plus the responsiveness in the group
>>> starting an application, plus the frame drop in the group streaming
>>> the movie.  In addition, we can measure the bandwidth/iops enjoyed by
>>> each group, plus, of course, the aggregate throughput of the whole
>>> system.  In particular we could compare results with throttling, BFQ,
>>> and CFQ.
>>> 
>>> Then we could write resulting numbers on the stone, and stick to them
>>> until something proves them wrong.
>>> 
>>> What do you (or others) think about it?
>> 
>> That sounds great and yeah it's lame that we didn't start with that.
>> Shaohua, would it be difficult to compare how bfq performs against
>> blk-throttle?
> 
> I had a test of BFQ.

Thank you very much for testing BFQ!

> I'm using BFQ found at
> http://algogroup.unimore.it/people/paolo/disk_sched/sources.php. version is
> 4.7.0-v8r3.

That's the latest stable version.  The development version [1] already
contains further improvements for fairness, latency and throughput.
It is however still a release candidate.

[1] https://github.com/linusw/linux-bfq/tree/bfq-v8

> It's a LSI SSD, queue depth 32. I use default setting. fio script
> is:
> 
> [global]
> ioengine=libaio
> direct=1
> readwrite=randread
> bs=4k
> runtime=60
> time_based=1
> file_service_type=random:36
> overwrite=1
> thread=0
> group_reporting=1
> filename=/dev/sdb
> iodepth=1
> numjobs=8
> 
> [groupA]
> prio=2
> 
> [groupB]
> new_group
> prio=6
> 
> I'll change iodepth, numjobs and prio in different tests. result unit is MB/s.
> 
> iodepth=1 numjobs=1 prio 4:4
> CFQ: 28:28 BFQ: 21:21 deadline: 29:29
> 
> iodepth=8 numjobs=1 prio 4:4
> CFQ: 162:162 BFQ: 102:98 deadline: 205:205
> 
> iodepth=1 numjobs=8 prio 4:4
> CFQ: 157:157 BFQ: 81:92 deadline: 196:197
> 
> iodepth=1 numjobs=1 prio 2:6
> CFQ: 26.7:27.6 BFQ: 20:6 deadline: 29:29
> 
> iodepth=8 numjobs=1 prio 2:6
> CFQ: 166:174 BFQ: 139:72  deadline: 202:202
> 
> iodepth=1 numjobs=8 prio 2:6
> CFQ: 148:150 BFQ: 90:77 deadline: 198:197
> 
> CFQ isn't fair at all. BFQ is very good in this side, but has poor throughput
> even prio is the default value.
> 

Throughput is lower with BFQ for two reasons.

First, you certainly left the low_latency in its default state, i.e.,
on.  As explained, e.g., here [2], low_latency mode is totally geared
towards maximum responsiveness and minimum latency for soft real-time
applications (e.g., video players).  To achieve this goal, BFQ is
willing to perform more idling, when necessary.  This lowers
throughput (I'll get back on this at the end of the discussion of the
second reason).

The second, most important reason, is that a minimum of idling is the
*only* way to achieve differentiated bandwidth distribution, as you
requested by setting different ioprios.  I stress that this constraint
is not a technological accident, but a intrinsic, logical necessity.
The proof is simple, and if the following explanation is too boring or
confusing, I can show it to you with any trace of sync I/O.

First, to provide differentiated service, you need per-process
scheduling, i.e., schedulers in which there is a separate queue
associated with each process.  Now, let A be the process with higher
weight (ioprio), and B the process with lower weight.  Both processes
are sync, thus, by definition, they issue requests as follows: a few
requests (probably two, or a little bit more with larger iodepth),
then a little break to wait for request completion, then the next
small batch and so on.  For each process, the queue associated with
the process (in the scheduler) is necessarily empty on the break.  As
a consequence, if there is no idling, then every time A reaches its
break, the scheduler has only the option to switch to B (which is
extremely likely to have pending requests).

The service pattern of the processes then unavoidably becomes:

A B A B A B ...

where each letter represents a full small batch served for the
process.  That is, 50% of the bw for each process, and complete loss
of control on the desired bandwidth distribution.

So, to sum up, the reason why BFQ achieves a lower total bw is that it
behaves in the only correct way to respect weights with sync I/O,
i.e., it performs a little idling.  If low_latency is on, then BFQ
increases idling further, and this may be have caused further bw loss
in your test (but this varies greatly with devices, so you can
discover it only by trying).

The bottom line is that if you do want to achieve differentiation with
sync I/O, you have to pay a price in terms of bw, because of idling.
Actually, the recent preemption mechanism that I have introduced in
BFQ is proving so effective in preserving differentiation, that I'm
tempted to try some almost idleness solution.  A little of accuracy
should however be sacrificed.  Anyway, this is still work in progress.

Thank you very much,
Paolo

[2] http://algogroup.unimore.it/people/paolo/disk_sched/description.php

> Thanks,
> Shaohua
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--
Paolo Valente
Algogroup
Dipartimento di Scienze Fisiche, Informatiche e Matematiche
Via Campi 213/B
41125 Modena - Italy
http://algogroup.unimore.it/people/paolo/