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Date:	Fri, 19 Aug 2011 10:55:02 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	Tao Ma <tm@....ma>, "Ted Ts'o" <tytso@....edu>,
	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: DIO process stuck apparently due to dioread_nolock (3.0)

On Fri, Aug 19, 2011 at 12:05 AM, Michael Tokarev <mjt@....msk.ru> wrote:
> On 19.08.2011 07:18, Tao Ma wrote:
>> Hi Michael,
>> On 08/18/2011 02:49 PM, Michael Tokarev wrote:
> []
>>> What about current situation, how do you think - should it be ignored
>>> for now, having in mind that dioread_nolock isn't used often (but it
>>> gives _serious_ difference in read speed), or, short term, fix this
>>> very case which have real-life impact already, while implementing a
>>> long-term solution?
>
>> So could you please share with us how you test and your test result
>> with/without dioread_nolock? A quick test with fio and intel ssd does't
>> see much improvement here.
>
> I used my home-grown quick-n-dirty microbenchmark for years to measure
> i/o subsystem performance.  Here are the results from 3.0 kernel on
> some Hitachi NAS (FC, on brocade adaptors), 14-drive raid10 array.
>
> The numbers are all megabytes/sec transferred (read or written), summed
> for all threads.  Leftmost column is the block size; next column is the
> number of concurrent threads of the same type.  And the columns are
> tests: linear read, random read, linear write, random write, and
> concurrent random read and write.
>
> For a raw device:
>
> BlkSz Trd linRd rndRd linWr rndWr  rndR/W
>   4k   1  18.3   0.8  14.5   9.6   0.1/  9.1
>        4         2.5         9.4   0.4/  8.4
>       32        10.0         9.3   4.7/  5.4
>  16k   1  59.4   2.5  49.9  35.7   0.3/ 34.7
>        4        10.3        36.1   1.5/ 31.4
>       32        38.5        36.2  17.5/ 20.4
>  64k   1 118.4   9.1 136.0 106.5   1.1/105.8
>        4        37.7       108.5   4.7/102.6
>       32       153.0       108.5  57.9/ 73.3
>  128k   1 125.9  16.5 138.8 125.8   1.1/125.6
>        4        68.7       128.7   6.3/122.8
>       32       277.0       128.7  70.3/ 98.6
> 1024k   1  89.9  81.2 138.9 134.4   5.0/132.3
>        4       254.7       137.6  19.2/127.1
>       32       390.7       137.5 117.2/ 90.1
>
> For ext4fs, 1Tb file, default mount options:
>
> BlkSz Trd linRd rndRd linWr rndWr  rndR/W
>   4k   1  15.7   0.6  15.4   9.4   0.0/  9.0
>        4         2.6         9.3   0.0/  8.9
>       32        10.0         9.3   0.0/  8.9
>  16k   1  47.6   2.5  53.2  34.6   0.1/ 33.6
>        4        10.2        34.6   0.0/ 33.5
>       32        39.9        34.8   0.1/ 33.6
>  64k   1 100.5   9.0 137.0 106.2   0.2/105.8
>        4        37.8       107.8   0.1/106.1
>       32       153.9       107.8   0.2/105.9
>  128k   1 115.4  16.3 138.6 125.2   0.3/125.3
>        4        68.8       127.8   0.2/125.6
>       32       274.6       127.8   0.2/126.2
> 1024k   1 124.5  54.2 138.9 133.6   1.0/133.3
>        4       159.5       136.6   0.2/134.3
>       32       349.7       136.5   0.3/133.6
>
> And for a 1tb file on ext4fs with dioread_nolock:
>
> BlkSz Trd linRd rndRd linWr rndWr  rndR/W
>   4k   1  15.7   0.6  14.6   9.4   0.1/  9.0
>        4         2.6         9.4   0.3/  8.6
>       32        10.0         9.4   4.5/  5.3
>  16k   1  50.9   2.4  56.7  36.0   0.3/ 35.2
>        4        10.1        36.4   1.5/ 34.6
>       32        38.7        36.4  17.3/ 21.0
>  64k   1  95.2   8.9 136.5 106.8   1.0/106.3
>        4        37.7       108.4   5.2/103.3
>       32       152.7       108.6  57.4/ 74.0
>  128k   1 115.1  16.3 138.8 125.8   1.2/126.4
>        4        68.9       128.5   5.7/124.0
>       32       276.1       128.6  70.8/ 98.5
> 1024k   1 128.5  81.9 138.9 134.4   5.1/132.3
>        4       253.4       137.4  19.1/126.8
>       32       385.1       137.4 111.7/ 92.3
>
> These are complete test results.  First 4 result
> columns are merely identical, the difference is
> within last column.  Here they are together:
>
> BlkSz Trd     Raw      Ext4nolock  Ext4dflt
>   4k   1   0.1/  9.1   0.1/  9.0  0.0/  9.0
>        4   0.4/  8.4   0.3/  8.6  0.0/  8.9
>       32   4.7/  5.4   4.5/  5.3  0.0/  8.9
>  16k   1   0.3/ 34.7   0.3/ 35.2  0.1/ 33.6
>        4   1.5/ 31.4   1.5/ 34.6  0.0/ 33.5
>       32  17.5/ 20.4  17.3/ 21.0  0.1/ 33.6
>  64k   1   1.1/105.8   1.0/106.3  0.2/105.8
>        4   4.7/102.6   5.2/103.3  0.1/106.1
>       32  57.9/ 73.3  57.4/ 74.0  0.2/105.9
>  128k   1   1.1/125.6   1.2/126.4  0.3/125.3
>        4   6.3/122.8   5.7/124.0  0.2/125.6
>       32  70.3/ 98.6  70.8/ 98.5  0.2/126.2
> 1024k   1   5.0/132.3   5.1/132.3  1.0/133.3
>        4  19.2/127.1  19.1/126.8  0.2/134.3
>       32 117.2/ 90.1 111.7/ 92.3  0.3/133.6
>
> Ext4 with dioread_nolock (middle column) behaves close to
> raw device.  But default ext4 greatly prefers writes over
> reads, reads are almost non-existent.
>
> This is, again, more or less a microbenchmark.  Where it
> come from is my attempt to simulate an (oracle) database
> workload (many years ago, when larger and more standard
> now benchmarks weren't (freely) available).  And there,
> on a busy DB, the difference is quite well-visible.
> In short, any writer makes all readers to wait.  Once
> we start writing something, all users immediately notice.
> With dioread_nolock they don't complain anymore.
>
> There's some more background around this all.  Right
> now I'm evaluating a new machine for our current database.
> Old hardware had 2Gb RAM so it had _significant_ memory
> pressure, and lots of stuff weren't able to be cached.
> New machine has 128Gb of RAM, which will ensure that
> all important stuff is in cache.  So the effect of this
> read/write disbalance will be much less visible.
>
> For example, we've a dictionary (several tables) with
> addresses - towns, streets, even buildings.  When they
> enter customer information they search in these dicts.
> With current 2Gb memory thses dictionaries can't be
> kept in memory, so they gets read from disk again every
> time someone enters customer information, and this is
> what they do all the time.  So no doubt disk access is
> very important here.
>
> On a new hardware, obviously, all these dictionaries will
> be in memory after first access, so even if all reads will
> wait till any write completes, it wont be as dramatic as
> it is now.
>
> That to say, -- maybe I'm really paying too much attention
> for a wrong problem.  So far, on a new machine, I don't see
> actual noticeable difference between dioread_nolock and
> without that option.
>
> (BTW, I found no way to remount a filesystem to EXclude
> that option, I have to umount and mount it in order to
> switch from using dioread_nolock to not using it.  Is
> there a way?)
I think the command to do this is:
mount -o remount,dioread_lock /dev/xxx <mountpoint>

Now looking at this, I guess it is not very intuitive that the option to
turn off dioread_nolock is dioread_lock instead of nodioread_nolock,
but nodioread_nolock does look ugly. Maybe we should try to support
both ways.

Jiaying
>
> Thanks,
>
> /mjt
>
>> We are based on RHEL6, and dioread_nolock isn't there by now and a large
>> number of our product system use direct read and buffer write. So if
>> your test proves to be promising, I guess our company can arrange some
>> resources to try to work it out.
>>
>> Thanks
>> Tao
>
>
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