Message-ID: <5194FCAC.1010300@ll.mit.edu>
Date:	Thu, 16 May 2013 11:35:08 -0400
From:	David Oostdyk <daveo@...mit.edu>
To:	"stan@...dwarefreak.com" <stan@...dwarefreak.com>
CC:	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xfs@....sgi.com" <xfs@....sgi.com>
Subject: Re: high-speed disk I/O is CPU-bound?

On 05/16/13 07:36, Stan Hoeppner wrote:
> On 5/15/2013 7:59 PM, Dave Chinner wrote:
>> [cc xfs list, seeing as that's where all the people who use XFS in
>> these sorts of configurations hang out. ]
>>
>> On Fri, May 10, 2013 at 10:04:44AM -0400, David Oostdyk wrote:
>>> Hello,
>>>
>>> I have a few relatively high-end systems with hardware RAIDs which
>>> are being used for recording systems, and I'm trying to get a better
>>> understanding of contiguous write performance.
>>>
>>> The hardware that I've tested with includes two high-end Intel
>>> E5-2600 and E5-4600 (~3GHz) series systems, as well as a slightly
>>> older Xeon 5600 system.  The JBODs include a 45x3.5" JBOD, a 28x3.5"
>>> JBOD (with either 7200RPM or 10kRPM SAS drives), and a 24x2.5" JBOD
>>> with 10kRPM drives.  I've tried LSI controllers (9285-8e, 9266-8i,
>>> as well as the integrated Intel LSI controllers) as well as Adaptec
>>> Series 7 RAID controllers (72405 and 71685).
> So, you have something like the following raw aggregate drive b/w,
> assuming average outer-inner track 120MB/s streaming write throughput
> per drive:
>
> 45 drives ~5.4 GB/s
> 28 drives ~3.4 GB/s
> 24 drives ~2.8 GB/s
>
> The two LSI HBAs you mention are PCIe 2.0 devices.  Note that PCIe 2.0
> x8 is limited to ~4GB/s each way.  If those 45 drives are connected to
> the 9285-8e via all 8 SAS lanes, you are still losing about 1/3rd of the
> aggregate drive b/w.  If they're connected to the 71685 via 8 lanes and
> this HBA is in a PCIe 3.0 slot then you're only losing about 600MB/s.
>
>>> Normally I'll setup the RAIDs as RAID60 and format them as XFS, but
>>> the exact RAID level, filesystem type, and even RAID hardware don't
>>> seem to matter very much from my observations (but I'm willing to
>>> try any suggestions).
> Lack of performance variability here tends to suggest your workloads are
> all streaming in nature, and/or your application profile isn't taking
> full advantage of the software stack and the hardware, i.e. insufficient
> parallelism, overlapping IOs, etc.  Or, see down below for another
> possibility.
>
> These are all current generation HBAs with fast multi-core ASICs and big
> write cache.  RAID6 parity writes even with high drive counts shouldn't
> significantly degrade large streaming write performance.  RMW workloads
> will still suffer substantially as usual due to rotational latencies.
> Fast ASICs can't solve this problem.
>
>> Document them. There's many ways to screw them up and get bad
>> performance.
> More detailed info always helps.
>
>>> As a basic benchmark, I have an application
>>> that simply writes the same buffer (say, 128MB) to disk repeatedly.
>>> Alternatively you could use the "dd" utility.  (For these
>>> benchmarks, I set /proc/sys/vm/dirty_bytes to 512M or lower, since
>>> these systems have a lot of RAM.)
>>>
>>> The basic observations are:
>>>
>>> 1.  "single-threaded" writes, either a file on the mounted
>>> filesystem or with a "dd" to the raw RAID device, seem to be limited
>>> to 1200-1400MB/sec.  These numbers vary slightly based on whether
>>> TurboBoost is affecting the writing process or not.  "top" will show
>>> this process running at 100% CPU.
>> Expected. You are using buffered IO. Write speed is limited by the
>> rate at which your user process can memcpy data into the page cache.
>>
>>> 2.  With two benchmarks running on the same device, I see aggregate
>>> write speeds of up to ~2.4GB/sec, which is closer to what I'd expect
>>> the drives of being able to deliver.  This can either be with two
>>> applications writing to separate files on the same mounted file
>>> system, or two separate "dd" applications writing to distinct
>>> locations on the raw device.
> 2.4GB/s is the interface limit of quad lane 6G SAS.  Coincidence?  If
> you've daisy chained the SAS expander backplanes within a server chassis
> (9266-8i/72405), or between external enclosures (9285-8e/71685), and
> have a single 4 lane cable (SFF-8087/8088/8643/8644) connected to your
> RAID card, this would fully explain the 2.4GB/s wall, regardless of how
> many parallel processes are writing, or any other software factor.
>
> But surely you already know this, and you're using more than one 4 lane
> cable.  Just covering all the bases here, due to seeing 2.4 GB/s as the
> stated wall.  This number is just too coincidental to ignore.

We definitely have two 4-lane cables being used, but this is an 
interesting coincidence.  I'd be surprised if anyone could really 
achieve the theoretical throughput on one cable, though.  We have one 
JBOD that only takes a single 4-lane cable, and we seem to cap out at 
closer to 1450MB/sec on that unit.  (This is just a single point of 
reference, and I don't have many tests where only one 4-lane cable was 
in use.)

>>> (Increasing the number of writers
>>> beyond two does not seem to increase aggregate performance; "top"
>>> will show both processes running at perhaps 80% CPU).
> So you're not referring to dd processes when you say "writers beyond
> two".  Otherwise you'd say "four" or "eight" instead of "both" processes.
>
>> Still using buffered IO, which means you are typically limited by
>> the rate at which the flusher thread can do writeback.
>>
>>> 3.  I haven't been able to find any tricks (lio_listio, multiple
>>> threads writing to distinct file offsets, etc) that seem to deliver
>>> higher write speeds when writing to a single file.  (This might be
>>> xfs-specific, though)
>> How about using direct IO? Single threaded direct IO will beslower
>> than buffered IO, but throughput should scale linearly with the
>> number of threads if the IO size is large enough (e.g. 32MB).
> Try this quick/dirty parallel write test using dd with O_DIRECT file
> based output using 1MB IOs.  It fires up 16 dd processes writing 16
> files in parallel, 4GB each.  This test should give a fairly accurate
> representation of real hardware throughput.  Sum the MB/s figures from
> all dd processes for the aggregate b/w.
>
> #!/bin/sh
> for i in {1..16}
> do
>      dd if=/dev/zero of=/XFS_dir/file.$i oflag=direct bs=1M count=4k &
> done
> wait
>
>>> 4.  Cheap tricks like making a software RAID0 of two hardware RAID
>>> devices does not deliver any improved performance for
>>> single-threaded writes.
> As Dave C points out, you'll never reach peak throughput with single
> threaded buffered IO.  You'd think it would be easy to hit peak write
> speed with a single 7.2k SATA drive using a single write thread.  Here's
> a salient demonstration of why this may not be the case.
>
> $ dd if=/dev/zero of=/XFS-mount/one-thread bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 17.8513 s, 58.7 MB/s
>
> Now a 4 thread variant of the script mentioned above:
>
> #!/bin/sh
> for i in {1..4}
> do
>      dd if=/dev/zero of=/XFS-mount/file.$i oflag=direct bs=1M count=512 &
> done
> wait
>
> $ test.sh
> 512+0 records in
> 512+0 records out
> 536870912 bytes (537 MB) copied, 20.3012 s, 26.4 MB/s
> 512+0 records in
> 512+0 records out
> 536870912 bytes (537 MB) copied, 20.3006 s, 26.4 MB/s
> 512+0 records in
> 512+0 records out
> 536870912 bytes (537 MB) copied, 20.3204 s, 26.4 MB/s
> 512+0 records in
> 512+0 records out
> 536870912 bytes (537 MB) copied, 20.324 s, 26.4 MB/s
>
> Single thread buffered write:	 59 MB/s
> Quad thread O_DIRECT write:	105 MB/s
>
> Again both targeting a single SATA disk.  I just ran these tests on a 13
> year old machine with dual 550MHz Celeron CPUs and 384MB of PC100 DRAM,
> vanilla kernel 3.2.6, deadline elevator.  The WD SATA disk is attached
> via a $20 USD Silicon Image 3512 SATA 150 32 bit PCI card lacking NCQ
> support.  The system bus is 33MHz/32 bit PCI only, 132MB/s peak, tested
> at 115MB/s net after PCI 2.1 protocol overhead.  I keep this system
> around for such demonstrations.  Note that the SATA card and drive are
> 10 years newer than the core system, acquired in 2009.
>
> On this machine the single thread buffered IO dd run reaches only some
> 51% of the net PCI throughput and eats 98% of one of the two 550MHz
> CPUs.  This is due to a number of factors including, but not limited to,
> memcpy as Dave C points out, tiny 128KB L2 cache, no L3, the fact that
> this platform performs snooping on the P6 bus, and other inefficiencies
> of the 440BX chipset.
>
> Now for the kicker.  Quad parallel dd direct IO reaches 92% of net PCI
> throughput with each dd process eating only 14% CPU, or 28% of each CPU
> total.  Its aggregate file write throughput into XFS is some 78% higher
> than single thread dd using buffered IO.
>
>> (Have not thoroughly tested this
>>> configuration fully with multiple writers, though.)
> You may not see a 78% bump with parallel O_DIRECT, but it should be
> substantial nonetheless.
>
>> Of course not - you are CPU bound and nothing you do to the storage
>> will change that.
> I'd agree 100% with Chinner if not for that pesky coincidental 2.4GB/s
> number reported as the "brick wall".  A little more info should clear
> this up.
>

You guys hit the nail on the head!  With O_DIRECT I can use a single 
writer thread and easily see the same throughput that I _ever_ saw in 
the multiple-writer case (~2.4GB/sec), and "top" shows the writer at 10% 
CPU usage.  I've modified my application to use O_DIRECT and it makes a 
world of difference.

[It's interesting that you see performance benefits for O_DIRECT even 
with a single SATA drive.  The reason it took me so long to test 
O_DIRECT in this case, is that I never saw any significant benefit from 
using it in the past.  But that is when I didn't have such fast storage, 
so I probably wasn't hitting the bottleneck with buffered I/O?]

So I have two systems, one with an LSI controller and one with an 
Adaptec 71685, each has two 4-lane cables going to 24 and 28 disks 
respectively, and they both are hitting about 2.4GB/sec.  I'm interested 
to test the Adaptec 74205 which is x8 3.0 and can connect six 4-lane 
cables directly to 24 drives.  That might shed some light on whether the 
2.4GB/sec "limit" is due to cable throughput, and I will follow up if 
that test proves interesting.

Thank you both for the suggestions!

- Dave O.



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