Date:	Tue, 16 Jun 2015 22:14:30 +0800
From:	juncheng bai <baijuncheng@...tedstack.com>
To:	Ilya Dryomov <idryomov@...il.com>
CC:	idryomov@...hat.com, Alex Elder <elder@...aro.org>,
	Josh Durgin <josh.durgin@...tank.com>,
	Guangliang Zhao <lucienchao@...il.com>, jeff@...zik.org,
	yehuda@...newdream.net, Sage Weil <sage@...dream.net>,
	elder@...tank.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ceph Development <ceph-devel@...r.kernel.org>
Subject: Re: [PATCH RFC] storage:rbd: make the size of request is equal to
 the, size of the object



On 2015/6/16 21:30, Ilya Dryomov wrote:
> On Tue, Jun 16, 2015 at 2:57 PM, juncheng bai
> <baijuncheng@...tedstack.com> wrote:
>>
>>
>> On 2015/6/16 16:37, Ilya Dryomov wrote:
>>>
>>> On Tue, Jun 16, 2015 at 6:28 AM, juncheng bai
>>> <baijuncheng@...tedstack.com> wrote:
>>>>
>>>>
>>>>
>>>> On 2015/6/15 22:27, Ilya Dryomov wrote:
>>>>>
>>>>>
>>>>> On Mon, Jun 15, 2015 at 4:23 PM, juncheng bai
>>>>> <baijuncheng@...tedstack.com> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 2015/6/15 21:03, Ilya Dryomov wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Mon, Jun 15, 2015 at 2:18 PM, juncheng bai
>>>>>>> <baijuncheng@...tedstack.com> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>     From 6213215bd19926d1063d4e01a248107dab8a899b Mon Sep 17 00:00:00
>>>>>>>> 2001
>>>>>>>> From: juncheng bai <baijuncheng@...tedstack.com>
>>>>>>>> Date: Mon, 15 Jun 2015 18:34:00 +0800
>>>>>>>> Subject: [PATCH] storage:rbd: make the size of request is equal to
>>>>>>>> the
>>>>>>>>      size of the object
>>>>>>>>
>>>>>>>> ensures that the merged size of request can achieve the size of
>>>>>>>> the object.
>>>>>>>> when merge a bio to request or merge a request to request, the
>>>>>>>> sum of the segment number of the current request and the segment
>>>>>>>> number of the bio is not greater than the max segments of the
>>>>>>>> request,
>>>>>>>> so the max size of request is 512k if the max segments of request is
>>>>>>>> BLK_MAX_SEGMENTS.
>>>>>>>>
>>>>>>>> Signed-off-by: juncheng bai <baijuncheng@...tedstack.com>
>>>>>>>> ---
>>>>>>>>      drivers/block/rbd.c | 2 ++
>>>>>>>>      1 file changed, 2 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>>>>>>> index 0a54c58..dec6045 100644
>>>>>>>> --- a/drivers/block/rbd.c
>>>>>>>> +++ b/drivers/block/rbd.c
>>>>>>>> @@ -3757,6 +3757,8 @@ static int rbd_init_disk(struct rbd_device
>>>>>>>> *rbd_dev)
>>>>>>>>             segment_size = rbd_obj_bytes(&rbd_dev->header);
>>>>>>>>             blk_queue_max_hw_sectors(q, segment_size / SECTOR_SIZE);
>>>>>>>>             blk_queue_max_segment_size(q, segment_size);
>>>>>>>> +       if (segment_size > BLK_MAX_SEGMENTS * PAGE_SIZE)
>>>>>>>> +               blk_queue_max_segments(q, segment_size / PAGE_SIZE);
>>>>>>>>             blk_queue_io_min(q, segment_size);
>>>>>>>>             blk_queue_io_opt(q, segment_size);
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> I made a similar patch on Friday, investigating blk-mq plugging issue
>>>>>>> reported by Nick.  My patch sets it to BIO_MAX_PAGES unconditionally -
>>>>>>> AFAIU there is no point in setting to anything bigger since the bios
>>>>>>> will be clipped to that number of vecs.  Given that BIO_MAX_PAGES is
>>>>>>> 256, this gives is 1M direct I/Os.
>>>>>>
>>>>>>
>>>>>>
>>>>>> Hi. For signal bio, the max number of bio_vec is BIO_MAX_PAGES, but a
>>>>>> request can be merged from multiple bios. We can see the below
>>>>>> function:
>>>>>> ll_back_merge_fn, ll_front_merge_fn and etc.
>>>>>> And I test in kernel 3.18 use this patch, and do:
>>>>>> echo 4096 > /sys/block/rbd0/queue/max_sectors_kb
>>>>>> We use systemtap to trace the request size, It is upto 4M.
>>>>>
>>>>>
>>>>>
>>>>> Kernel 3.18 is pre rbd blk-mq transition, which happened in 4.0.  You
>>>>> should test whatever patches you have with at least 4.0.
>>>>>
>>>>> Putting that aside, I must be missing something.  You'll get 4M
>>>>> requests on 3.18 both with your patch and without it, the only
>>>>> difference would be the size of bios being merged - 512k vs 1M.  Can
>>>>> you describe your test workload and provide before and after traces?
>>>>>
>>>> Hi. I update kernel version to 4.0.5. The test information as shown
>>>> below:
>>>> The base information:
>>>> 03:28:13-root@...ver-186:~$uname -r
>>>> 4.0.5
>>>>
>>>> My simple systemtap script:
>>>> probe module("rbd").function("rbd_img_request_create")
>>>> {
>>>>       printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>>>> }
>>>>
>>>> I use dd to execute the test case:
>>>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=1 oflag=direct
>>>>
>>>> Case one: Without patch
>>>> 03:30:23-root@...ver-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>>>> 4096
>>>> 03:30:35-root@...ver-186:~$cat /sys/block/rbd0/queue/max_segments
>>>> 128
>>>>
>>>> The output of systemtap for nornal data:
>>>> offset:0 length:524288
>>>> offset:524288 length:524288
>>>> offset:1048576 length:524288
>>>> offset:1572864 length:524288
>>>> offset:2097152 length:524288
>>>> offset:2621440 length:524288
>>>> offset:3145728 length:524288
>>>> offset:3670016 length:524288
>>>>
>>>> Case two:With patch
>>>> cat /sys/block/rbd0/queue/max_sectors_kb
>>>> 4096
>>>> 03:49:14-root@...ver-186:linux-4.0.5$cat
>>>> /sys/block/rbd0/queue/max_segments
>>>> 1024
>>>> The output of systemtap for nornal data:
>>>> offset:0 length:1048576
>>>> offset:1048576 length:1048576
>>>> offset:2097152 length:1048576
>>>> offset:3145728 length:1048576
>>>>
>>>> According to the test, you are right.
>>>> Because the blk-mq doesn't use any scheduling policy.
>>>> 03:52:13-root@...ver-186:linux-4.0.5$cat /sys/block/rbd0/queue/scheduler
>>>> none
>>>>
>>>> In previous versions of the kernel 4.0, the rbd use the defualt
>>>> scheduler:cfq
>>>>
>>>> So, I think that the blk-mq need to do more?
>>>
>>>
>>> There is no scheduler support in blk-mq as of now but your numbers
>>> don't have anything to do with that.  The current behaviour is a result
>>> of a bug in blk-mq.  It's fixed by [1], if you apply it you should see
>>> 4M requests with your stap script.
>>>
>>> [1] http://article.gmane.org/gmane.linux.kernel/1941750
>>>
>> Hi.
>> First, Let's look at the result in the kernel version 3.18
>> The function blk_limits_max_hw_sectors different implemention between 3.18
>> and 4.0+. We need do:
>> echo 4094 >/sys/block/rbd0/queue/max_sectors_kb
>>
>> The rbd device information:
>> 11:13:18-root@...ver-186:~$cat /sys/block/rbd0/queue/max_sectors_kb
>> 4094
>> 11:15:28-root@...ver-186:~$cat /sys/block/rbd0/queue/max_segments
>> 1024
>>
>> The test command:
>> dd if=/dev/zero of=/dev/rbd0 bs=4M count=1
>>
>> The simple stap script:
>> probe module("rbd").function("rbd_img_request_create")
>> {
>>      printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>> }
>>
>> The output from stap:
>> offset:0 length:4190208
>> offset:21474770944 length:4096
>>
>> Second, thanks for your patch [1].
>> I use the patch [1], and recompile the kernel.
>> The test information as shown below:
>> 12:26:12-root@...ver-186:$cat /sys/block/rbd0/queue/max_segments
>> 1024
>> 12:26:23-root@...ver-186:$cat /sys/block/rbd0/queue/max_sectors_kb
>> 4096
>>
>> The test command:
>> dd if=/dev/zero  of=/dev/rbd0 bs=4M count=2 oflag=direct
>>
>> The simple systemtap script:
>> probe module("rbd").function("rbd_img_request_create")
>> {
>>      printf("offset:%lu length:%lu\n", ulong_arg(2), ulong_arg(3));
>> }
>>
>> The output of systemtap for nornal data:
>> offset:0 length:4194304
>> offset:4194304 length:4194304
>> offset:21474770944 length:4096
>
> Sorry, I fail to see the purpose of the above tests.  The test commands
> differ, the kernels differ and it looks like you had your patch applied
> for both tests.  What I'm trying to get you to do is to show me some
> data that will back your claim (which your patch is based on):
>
>>
>> So, I think that the max_segments of request_limits should be divide the
>> object size by PAGE_SIZE.
>
> For that you need to use the same kernel and run the same workload.
> The only difference should be whether your patch is applied or not.
> I still think that setting rbd max_segments to anything above
> BIO_MAX_PAGES is bogus, but I'd be happy to be shown wrong on that
> since that would mean better performance, at least in some
> workloads.
>
Hi.
For cloned image, it will avoid doing copyup if the request size is
equal to the object size, I think that it is the key effect of this
patch.
The big request would result in overtime if the ceph backend is busy
or the network bandwidth is too low.
I suggest that add a module parameter to control the value which
decided by the user settings.

Thanks.
----
juncheng bai

> Thanks,
>
>                  Ilya
>
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