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[dm-devel] Re: IO scheduler based IO controller V10



On Fri, Sep 25, 2009 at 10:09:52AM +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 24 Sep 2009 14:33:15 -0700
> Andrew Morton <akpm linux-foundation org> wrote:
> > > Test5 (Fairness for async writes, Buffered Write Vs Buffered Write)
> > > ===================================================================
> > > Fairness for async writes is tricky and biggest reason is that async writes
> > > are cached in higher layers (page cahe) as well as possibly in file system
> > > layer also (btrfs, xfs etc), and are dispatched to lower layers not necessarily
> > > in proportional manner.
> > > 
> > > For example, consider two dd threads reading /dev/zero as input file and doing
> > > writes of huge files. Very soon we will cross vm_dirty_ratio and dd thread will
> > > be forced to write out some pages to disk before more pages can be dirtied. But
> > > not necessarily dirty pages of same thread are picked. It can very well pick
> > > the inode of lesser priority dd thread and do some writeout. So effectively
> > > higher weight dd is doing writeouts of lower weight dd pages and we don't see
> > > service differentation.
> > > 
> > > IOW, the core problem with buffered write fairness is that higher weight thread
> > > does not throw enought IO traffic at IO controller to keep the queue
> > > continuously backlogged. In my testing, there are many .2 to .8 second
> > > intervals where higher weight queue is empty and in that duration lower weight
> > > queue get lots of job done giving the impression that there was no service
> > > differentiation.
> > > 
> > > In summary, from IO controller point of view async writes support is there.
> > > Because page cache has not been designed in such a manner that higher 
> > > prio/weight writer can do more write out as compared to lower prio/weight
> > > writer, gettting service differentiation is hard and it is visible in some
> > > cases and not visible in some cases.
> > 
> > Here's where it all falls to pieces.
> > 
> > For async writeback we just don't care about IO priorities.  Because
> > from the point of view of the userspace task, the write was async!  It
> > occurred at memory bandwidth speed.
> > 
> > It's only when the kernel's dirty memory thresholds start to get
> > exceeded that we start to care about prioritisation.  And at that time,
> > all dirty memory (within a memcg?) is equal - a high-ioprio dirty page
> > consumes just as much memory as a low-ioprio dirty page.
> > 
> > So when balance_dirty_pages() hits, what do we want to do?
> > 
> > I suppose that all we can do is to block low-ioprio processes more
> > agressively at the VFS layer, to reduce the rate at which they're
> > dirtying memory so as to give high-ioprio processes more of the disk
> > bandwidth.
> > 
> > But you've gone and implemented all of this stuff at the io-controller
> > level and not at the VFS level so you're, umm, screwed.
> > 
> 
> I think I must support dirty-ratio in memcg layer. But not yet.
> I can't easily imagine how the system will work if both dirty-ratio and
> io-controller cgroup are supported.

IIUC, you are suggesting per memeory cgroup dirty ratio and writer will be 
throttled if dirty ratio is crossed.  makes sense to me. Just that io
controller and memory controller shall have to me mounted together.

Thanks
Vivek

> But considering use them as a set of
> cgroup, called containers(zone?), it's will not be bad, I think.
> 
> The final bottelneck queue for fairness in usual workload on usual (small)
> server will ext3's journal, I wonder ;)
> 
> Thanks,
> -Kame
> 
> 
> > Importantly screwed!  It's a very common workload pattern, and one
> > which causes tremendous amounts of IO to be generated very quickly,
> > traditionally causing bad latency effects all over the place.  And we
> > have no answer to this.
> > 
> > > Vanilla CFQ Vs IO Controller CFQ
> > > ================================
> > > We have not fundamentally changed CFQ, instead enhanced it to also support
> > > hierarchical io scheduling. In the process invariably there are small changes
> > > here and there as new scenarios come up. Running some tests here and comparing
> > > both the CFQ's to see if there is any major deviation in behavior.
> > > 
> > > Test1: Sequential Readers
> > > =========================
> > > [fio --rw=read --bs=4K --size=2G --runtime=30 --direct=1 --numjobs=<1 to 16> ]
> > > 
> > > IO scheduler: Vanilla CFQ
> > > 
> > > nr  Max-bdwidth Min-bdwidth Agg-bdwidth Max-latency 
> > > 1   35499KiB/s  35499KiB/s  35499KiB/s  19195 usec  
> > > 2   17089KiB/s  13600KiB/s  30690KiB/s  118K usec   
> > > 4   9165KiB/s   5421KiB/s   29411KiB/s  380K usec   
> > > 8   3815KiB/s   3423KiB/s   29312KiB/s  830K usec   
> > > 16  1911KiB/s   1554KiB/s   28921KiB/s  1756K usec  
> > > 
> > > IO scheduler: IO controller CFQ
> > > 
> > > nr  Max-bdwidth Min-bdwidth Agg-bdwidth Max-latency 
> > > 1   34494KiB/s  34494KiB/s  34494KiB/s  14482 usec  
> > > 2   16983KiB/s  13632KiB/s  30616KiB/s  123K usec   
> > > 4   9237KiB/s   5809KiB/s   29631KiB/s  372K usec   
> > > 8   3901KiB/s   3505KiB/s   29162KiB/s  822K usec   
> > > 16  1895KiB/s   1653KiB/s   28945KiB/s  1778K usec  
> > > 
> > > Test2: Sequential Writers
> > > =========================
> > > [fio --rw=write --bs=4K --size=2G --runtime=30 --direct=1 --numjobs=<1 to 16> ]
> > > 
> > > IO scheduler: Vanilla CFQ
> > > 
> > > nr  Max-bdwidth Min-bdwidth Agg-bdwidth Max-latency 
> > > 1   22669KiB/s  22669KiB/s  22669KiB/s  401K usec   
> > > 2   14760KiB/s  7419KiB/s   22179KiB/s  571K usec   
> > > 4   5862KiB/s   5746KiB/s   23174KiB/s  444K usec   
> > > 8   3377KiB/s   2199KiB/s   22427KiB/s  1057K usec  
> > > 16  2229KiB/s   556KiB/s    20601KiB/s  5099K usec  
> > > 
> > > IO scheduler: IO Controller CFQ
> > > 
> > > nr  Max-bdwidth Min-bdwidth Agg-bdwidth Max-latency 
> > > 1   22911KiB/s  22911KiB/s  22911KiB/s  37319 usec  
> > > 2   11752KiB/s  11632KiB/s  23383KiB/s  245K usec   
> > > 4   6663KiB/s   5409KiB/s   23207KiB/s  384K usec   
> > > 8   3161KiB/s   2460KiB/s   22566KiB/s  935K usec   
> > > 16  1888KiB/s   795KiB/s    21349KiB/s  3009K usec  
> > > 
> > > Test3: Random Readers
> > > =========================
> > > [fio --rw=randread --bs=4K --size=2G --runtime=30 --direct=1 --numjobs=1 to 16]
> > > 
> > > IO scheduler: Vanilla CFQ
> > > 
> > > nr  Max-bdwidth Min-bdwidth Agg-bdwidth Max-latency 
> > > 1   484KiB/s    484KiB/s    484KiB/s    22596 usec  
> > > 2   229KiB/s    196KiB/s    425KiB/s    51111 usec  
> > > 4   119KiB/s    73KiB/s     405KiB/s    2344 msec   
> > > 8   93KiB/s     23KiB/s     399KiB/s    2246 msec   
> > > 16  38KiB/s     8KiB/s      328KiB/s    3965 msec   
> > > 
> > > IO scheduler: IO Controller CFQ
> > > 
> > > nr  Max-bdwidth Min-bdwidth Agg-bdwidth Max-latency 
> > > 1   483KiB/s    483KiB/s    483KiB/s    29391 usec  
> > > 2   229KiB/s    196KiB/s    426KiB/s    51625 usec  
> > > 4   132KiB/s    88KiB/s     417KiB/s    2313 msec   
> > > 8   79KiB/s     18KiB/s     389KiB/s    2298 msec   
> > > 16  43KiB/s     9KiB/s      327KiB/s    3905 msec   
> > > 
> > > Test4: Random Writers
> > > =====================
> > > [fio --rw=randwrite --bs=4K --size=2G --runtime=30 --direct=1 --numjobs=1 to 16]
> > > 
> > > IO scheduler: Vanilla CFQ
> > > 
> > > nr  Max-bdwidth Min-bdwidth Agg-bdwidth Max-latency 
> > > 1   14641KiB/s  14641KiB/s  14641KiB/s  93045 usec  
> > > 2   7896KiB/s   1348KiB/s   9245KiB/s   82778 usec  
> > > 4   2657KiB/s   265KiB/s    6025KiB/s   216K usec   
> > > 8   951KiB/s    122KiB/s    3386KiB/s   1148K usec  
> > > 16  66KiB/s     22KiB/s     829KiB/s    1308 msec   
> > > 
> > > IO scheduler: IO Controller CFQ
> > > 
> > > nr  Max-bdwidth Min-bdwidth Agg-bdwidth Max-latency 
> > > 1   14454KiB/s  14454KiB/s  14454KiB/s  74623 usec  
> > > 2   4595KiB/s   4104KiB/s   8699KiB/s   135K usec   
> > > 4   3113KiB/s   334KiB/s    5782KiB/s   200K usec   
> > > 8   1146KiB/s   95KiB/s     3832KiB/s   593K usec   
> > > 16  71KiB/s     29KiB/s     814KiB/s    1457 msec   
> > > 
> > > Notes:
> > >  - Does not look like that anything has changed significantly.
> > > 
> > > Previous versions of the patches were posted here.
> > > ------------------------------------------------
> > > 
> > > (V1) http://lkml.org/lkml/2009/3/11/486
> > > (V2) http://lkml.org/lkml/2009/5/5/275
> > > (V3) http://lkml.org/lkml/2009/5/26/472
> > > (V4) http://lkml.org/lkml/2009/6/8/580
> > > (V5) http://lkml.org/lkml/2009/6/19/279
> > > (V6) http://lkml.org/lkml/2009/7/2/369
> > > (V7) http://lkml.org/lkml/2009/7/24/253
> > > (V8) http://lkml.org/lkml/2009/8/16/204
> > > (V9) http://lkml.org/lkml/2009/8/28/327
> > > 
> > > Thanks
> > > Vivek
> > --
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