Date:	Mon, 9 Jan 2012 09:16:15 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	linux-kernel@...r.kernel.org, axboe@...nel.dk, vgoyal@...hat.com,
	jmoyer@...hat.com
Subject: Re: [RFC 0/3]block: An IOPS based ioscheduler

On Fri, Jan 06, 2012 at 01:12:29PM +0800, Shaohua Li wrote:
> On Thu, 2012-01-05 at 14:50 +0800, Shaohua Li wrote:
> > On Wed, 2012-01-04 at 18:19 +1100, Dave Chinner wrote:
> > > On Wed, Jan 04, 2012 at 02:53:37PM +0800, Shaohua Li wrote:
> > > > An IOPS based I/O scheduler
> > > > 
> > > > Flash based storage has some different characteristics against rotate disk.
> > > > 1. no I/O seek.
> > > > 2. read and write I/O cost usually is much different.
> > > > 3. Time which a request takes depends on request size.
> > > > 4. High throughput and IOPS, low latency.
> > > > 
> > > > CFQ iosched does well for rotate disk, for example fair dispatching, idle
> > > > for sequential read. It also has optimization for flash based storage (for
> > > > item 1 above), but overall it's not designed for flash based storage. It's
> > > > a slice based algorithm. Since flash based storage request cost is very
> > > > low, and drive has big queue_depth is quite popular now which makes
> > > > dispatching cost even lower, CFQ's slice accounting (jiffy based)
> > > > doesn't work well. CFQ doesn't consider above item 2 & 3.
> > > > 
> > > > FIOPS (Fair IOPS) ioscheduler is trying to fix the gaps. It's IOPS based, so
> > > > only targets for drive without I/O seek. It's quite similar like CFQ, but
> > > > the dispatch decision is made according to IOPS instead of slice.
> > > > 
> > > > The algorithm is simple. Drive has a service tree, and each task lives in
> > > > the tree. The key into the tree is called vios (virtual I/O). Every request
> > > > has vios, which is calculated according to its ioprio, request size and so
> > > > on. Task's vios is the sum of vios of all requests it dispatches. FIOPS
> > > > always selects task with minimum vios in the service tree and let the task
> > > > dispatch request. The dispatched request's vios is then added to the task's
> > > > vios and the task is repositioned in the sevice tree.
> > > > 
> > > > The series are orgnized as:
> > > > Patch 1: separate CFQ's io context management code. FIOPS will use it too.
> > > > Patch 2: The core FIOPS.
> > > > Patch 3: request read/write vios scale. This demontrates how the vios scale.
> > > > 
> > > > To make the code simple for easy view, some scale code isn't included here,
> > > > some not implementated yet.
> > > > 
> > > > TODO:
> > > > 1. ioprio support (have patch already)
> > > > 2. request size vios scale
> > > > 3. cgroup support
> > > > 4. tracing support
> > > > 5. automatically select default iosched according to QUEUE_FLAG_NONROT.
> > > > 
> > > > Comments and suggestions are welcome!
> > > 
> > > Benchmark results?
> > I didn't have data yet. The patches are still in earlier stage, I want
> > to focus on the basic idea first.
> since you asked, I tested in a 4 socket machine with 12 X25M SSD jbod,
> fs is ext4.
> 
> workload		percentage change with fiops against cfq
> fio_sync_read_4k        -2
> fio_mediaplay_64k       0
> fio_mediaplay_128k      0
> fio_mediaplay_rr_64k    0
> fio_sync_read_rr_4k     0
> fio_sync_write_128k     0
> fio_sync_write_64k      -1
> fio_sync_write_4k       -2
> fio_sync_write_64k_create       0
> fio_sync_write_rr_64k_create    0
> fio_sync_write_128k_create      0
> fio_aio_randread_4k     -4
> fio_aio_randread_64k    0
> fio_aio_randwrite_4k    1
> fio_aio_randwrite_64k   0
> fio_aio_randrw_4k       -1
> fio_aio_randrw_64k      0
> fio_tpch        9
> fio_tpcc        0
> fio_mmap_randread_4k    -1
> fio_mmap_randread_64k   1
> fio_mmap_randread_1k    -8
> fio_mmap_randwrite_4k   35
> fio_mmap_randwrite_64k  22
> fio_mmap_randwrite_1k   28
> fio_mmap_randwrite_4k_halfbusy  24
> fio_mmap_randrw_4k      23
> fio_mmap_randrw_64k     4
> fio_mmap_randrw_1k      22
> fio_mmap_randrw_4k_halfbusy     35
> fio_mmap_sync_read_4k   0
> fio_mmap_sync_read_64k  -1
> fio_mmap_sync_read_128k         -1
> fio_mmap_sync_read_rr_64k       5
> fio_mmap_sync_read_rr_4k        3
> 
> The fio_mmap_randread_1k has regression against 3.2-rc7, but no
> regression against 3.2-rc6 kernel, still checking why. The fiops has
> improvement for read/write mixed workload. CFQ is known not good for
> read/write mixed workload.

Numbers like this are meaningless without knowing what the hardware
capability is and how the numbers compare to that raw capability.
They tell me only mmap based random write improves in
performance, and only one specific type of random write improves,
not all types.

That raises more questions that it answers: why do AIO based random
writes not go any faster? Is that because even with CFQ, AIO based
random writes saturate the device?  i.e. is AIO based IO that much
faster than mmap based IO that there is no scope for improvement on
your hardware?

You need to present raw numbers and give us some idea of how close
those numbers are to raw hardware capability for us to have any idea
what improvements these numbers actually demonstrate.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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