```All of lore.kernel.org
help / color / mirror / Atom feed```
```From: Wu Fengguang <fengguang.wu@intel.com>
To: Vivek Goyal <vgoyal@redhat.com>
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@lst.de>,
Dave Chinner <david@fromorbit.com>,
Minchan Kim <minchan.kim@gmail.com>,
Andrea Righi <arighi@develer.com>, linux-mm <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/5] writeback: dirty position control
Date: Wed, 24 Aug 2011 08:12:58 +0800	[thread overview]
Message-ID: <20110824001257.GA6349@localhost> (raw)

>
> 	balanced_rate = task_ratelimit_200ms * write_bw / dirty_rate
>
> without explaining why following will not work.
>
> 	balanced_rate_(i+1) = balance_rate(i) * write_bw / dirty_rate

Thanks for asking that, it's probably the root of confusions, so let

It's actually pretty simple to explain this equation:

write_bw
balanced_rate = task_ratelimit_200ms * ----------       (1)
dirty_rate

for the past 200ms, we are going to measure the overall bdi dirty rate

dirty_rate = N * task_ratelimit_200ms                   (2)

put (2) into (1) we get

balanced_rate = write_bw / N                            (3)

So equation (1) is the right estimation to get the desired target (3).

As for

write_bw
balanced_rate_(i+1) = balanced_rate_(i) * ----------    (4)
dirty_rate

Let's compare it with the "expanded" form of (1):

write_bw
balanced_rate_(i+1) = balanced_rate_(i) * pos_ratio * ----------      (5)
dirty_rate

So the difference lies in pos_ratio.

Believe it or not, it's exactly the seemingly use of pos_ratio that
makes (5) independent(*) of the position control.

Why? Look at (4), assume the system is in a state

- dirty rate is already balanced, ie. balanced_rate_(i) = write_bw / N
- dirty position is not balanced, for example pos_ratio = 0.5

balance_dirty_pages() will be rate limiting each tasks at half the
balanced dirty rate, yielding a measured

dirty_rate = write_bw / 2                               (6)

Put (6) into (4), we get

balanced_rate_(i+1) = balanced_rate_(i) * 2
= (write_bw / N) * 2

That means, any position imbalance will lead to balanced_rate
estimation errors if we follow (4). Whereas if (1)/(5) is used, we
always get the right balanced dirty ratelimit value whether or not
(pos_ratio == 1.0), hence make the rate estimation independent(*) of
dirty position control.

(*) independent as in real values, not the seemingly relations in equation

Thanks,
Fengguang
```

`WARNING: multiple messages have this Message-ID (diff)`
```From: Wu Fengguang <fengguang.wu@intel.com>
To: Vivek Goyal <vgoyal@redhat.com>
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@lst.de>,
Dave Chinner <david@fromorbit.com>,
Minchan Kim <minchan.kim@gmail.com>,
Andrea Righi <arighi@develer.com>, linux-mm <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/5] writeback: dirty position control
Date: Wed, 24 Aug 2011 08:12:58 +0800	[thread overview]
Message-ID: <20110824001257.GA6349@localhost> (raw)

>
> 	balanced_rate = task_ratelimit_200ms * write_bw / dirty_rate
>
> without explaining why following will not work.
>
> 	balanced_rate_(i+1) = balance_rate(i) * write_bw / dirty_rate

Thanks for asking that, it's probably the root of confusions, so let

It's actually pretty simple to explain this equation:

write_bw
balanced_rate = task_ratelimit_200ms * ----------       (1)
dirty_rate

for the past 200ms, we are going to measure the overall bdi dirty rate

dirty_rate = N * task_ratelimit_200ms                   (2)

put (2) into (1) we get

balanced_rate = write_bw / N                            (3)

So equation (1) is the right estimation to get the desired target (3).

As for

write_bw
balanced_rate_(i+1) = balanced_rate_(i) * ----------    (4)
dirty_rate

Let's compare it with the "expanded" form of (1):

write_bw
balanced_rate_(i+1) = balanced_rate_(i) * pos_ratio * ----------      (5)
dirty_rate

So the difference lies in pos_ratio.

Believe it or not, it's exactly the seemingly use of pos_ratio that
makes (5) independent(*) of the position control.

Why? Look at (4), assume the system is in a state

- dirty rate is already balanced, ie. balanced_rate_(i) = write_bw / N
- dirty position is not balanced, for example pos_ratio = 0.5

balance_dirty_pages() will be rate limiting each tasks at half the
balanced dirty rate, yielding a measured

dirty_rate = write_bw / 2                               (6)

Put (6) into (4), we get

balanced_rate_(i+1) = balanced_rate_(i) * 2
= (write_bw / N) * 2

That means, any position imbalance will lead to balanced_rate
estimation errors if we follow (4). Whereas if (1)/(5) is used, we
always get the right balanced dirty ratelimit value whether or not
(pos_ratio == 1.0), hence make the rate estimation independent(*) of
dirty position control.

(*) independent as in real values, not the seemingly relations in equation

Thanks,
Fengguang

--
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```

```next prev parent reply	other threads:[~2011-08-24  0:13 UTC|newest]

Thread overview: 305+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-08-06  8:44 [PATCH 0/5] IO-less dirty throttling v8 Wu Fengguang
2011-08-06  8:44 ` Wu Fengguang
2011-08-06  8:44 ` Wu Fengguang
2011-08-06  8:44 ` [PATCH 1/5] writeback: account per-bdi accumulated dirtied pages Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-06  8:44 ` [PATCH 2/5] writeback: dirty position control Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-08 13:46   ` Peter Zijlstra
2011-08-08 13:46     ` Peter Zijlstra
2011-08-08 13:46     ` Peter Zijlstra
2011-08-08 14:11     ` Wu Fengguang
2011-08-08 14:11       ` Wu Fengguang
2011-08-08 14:31       ` Peter Zijlstra
2011-08-08 14:31         ` Peter Zijlstra
2011-08-08 14:31         ` Peter Zijlstra
2011-08-08 22:47         ` Wu Fengguang
2011-08-08 22:47           ` Wu Fengguang
2011-08-09  9:31           ` Peter Zijlstra
2011-08-09  9:31             ` Peter Zijlstra
2011-08-09  9:31             ` Peter Zijlstra
2011-08-10 12:28             ` Wu Fengguang
2011-08-10 12:28               ` Wu Fengguang
2011-08-08 14:41       ` Peter Zijlstra
2011-08-08 14:41         ` Peter Zijlstra
2011-08-08 14:41         ` Peter Zijlstra
2011-08-08 23:05         ` Wu Fengguang
2011-08-08 23:05           ` Wu Fengguang
2011-08-09 10:32           ` Peter Zijlstra
2011-08-09 10:32             ` Peter Zijlstra
2011-08-09 10:32             ` Peter Zijlstra
2011-08-09 17:20           ` Peter Zijlstra
2011-08-09 17:20             ` Peter Zijlstra
2011-08-09 17:20             ` Peter Zijlstra
2011-08-10 22:34             ` Jan Kara
2011-08-10 22:34               ` Jan Kara
2011-08-11  2:29               ` Wu Fengguang
2011-08-11  2:29                 ` Wu Fengguang
2011-08-11 11:14                 ` Jan Kara
2011-08-11 11:14                   ` Jan Kara
2011-08-16  8:35                   ` Wu Fengguang
2011-08-16  8:35                     ` Wu Fengguang
2011-08-12 13:19             ` Wu Fengguang
2011-08-12 13:19               ` Wu Fengguang
2011-08-10 21:40           ` Vivek Goyal
2011-08-10 21:40             ` Vivek Goyal
2011-08-16  8:55             ` Wu Fengguang
2011-08-16  8:55               ` Wu Fengguang
2011-08-11 22:56           ` Peter Zijlstra
2011-08-11 22:56             ` Peter Zijlstra
2011-08-11 22:56             ` Peter Zijlstra
2011-08-12  2:43             ` Wu Fengguang
2011-08-12  2:43               ` Wu Fengguang
2011-08-12  3:18               ` Wu Fengguang
2011-08-12  5:45               ` Wu Fengguang
2011-08-12  5:45                 ` Wu Fengguang
2011-08-12  9:45                 ` Peter Zijlstra
2011-08-12  9:45                   ` Peter Zijlstra
2011-08-12  9:45                   ` Peter Zijlstra
2011-08-12 11:07                   ` Wu Fengguang
2011-08-12 11:07                     ` Wu Fengguang
2011-08-12 12:17                     ` Peter Zijlstra
2011-08-12 12:17                       ` Peter Zijlstra
2011-08-12 12:17                       ` Peter Zijlstra
2011-08-12  9:47               ` Peter Zijlstra
2011-08-12  9:47                 ` Peter Zijlstra
2011-08-12  9:47                 ` Peter Zijlstra
2011-08-12 11:11                 ` Wu Fengguang
2011-08-12 11:11                   ` Wu Fengguang
2011-08-12 12:54           ` Peter Zijlstra
2011-08-12 12:54             ` Peter Zijlstra
2011-08-12 12:54             ` Peter Zijlstra
2011-08-12 12:59             ` Wu Fengguang
2011-08-12 12:59               ` Wu Fengguang
2011-08-12 13:08               ` Peter Zijlstra
2011-08-12 13:08                 ` Peter Zijlstra
2011-08-12 13:08                 ` Peter Zijlstra
2011-08-12 13:04           ` Peter Zijlstra
2011-08-12 13:04             ` Peter Zijlstra
2011-08-12 13:04             ` Peter Zijlstra
2011-08-12 14:20             ` Wu Fengguang
2011-08-12 14:20               ` Wu Fengguang
2011-08-22 15:38               ` Peter Zijlstra
2011-08-22 15:38                 ` Peter Zijlstra
2011-08-22 15:38                 ` Peter Zijlstra
2011-08-23  3:40                 ` Wu Fengguang
2011-08-23  3:40                   ` Wu Fengguang
2011-08-23 10:01                   ` Peter Zijlstra
2011-08-23 10:01                     ` Peter Zijlstra
2011-08-23 10:01                     ` Peter Zijlstra
2011-08-23 14:15                     ` Wu Fengguang
2011-08-23 14:15                       ` Wu Fengguang
2011-08-23 17:47                       ` Vivek Goyal
2011-08-23 17:47                         ` Vivek Goyal
2011-08-24  0:12                         ` Wu Fengguang [this message]
2011-08-24  0:12                           ` Wu Fengguang
2011-08-24 16:12                           ` Peter Zijlstra
2011-08-24 16:12                             ` Peter Zijlstra
2011-08-26  0:18                             ` Wu Fengguang
2011-08-26  0:18                               ` Wu Fengguang
2011-08-26  9:04                               ` Peter Zijlstra
2011-08-26  9:04                                 ` Peter Zijlstra
2011-08-26 10:04                                 ` Wu Fengguang
2011-08-26 10:04                                   ` Wu Fengguang
2011-08-26 10:42                                   ` Peter Zijlstra
2011-08-26 10:42                                     ` Peter Zijlstra
2011-08-26 10:52                                     ` Wu Fengguang
2011-08-26 10:52                                       ` Wu Fengguang
2011-08-26 11:26                                   ` Wu Fengguang
2011-08-26 12:11                                     ` Peter Zijlstra
2011-08-26 12:11                                       ` Peter Zijlstra
2011-08-26 12:20                                       ` Wu Fengguang
2011-08-26 12:20                                         ` Wu Fengguang
2011-08-26 13:13                                         ` Wu Fengguang
2011-08-26 13:18                                           ` Peter Zijlstra
2011-08-26 13:18                                             ` Peter Zijlstra
2011-08-26 13:24                                             ` Wu Fengguang
2011-08-26 13:24                                               ` Wu Fengguang
2011-08-24 18:00                           ` Vivek Goyal
2011-08-24 18:00                             ` Vivek Goyal
2011-08-25  3:19                             ` Wu Fengguang
2011-08-25  3:19                               ` Wu Fengguang
2011-08-25 22:20                               ` Vivek Goyal
2011-08-25 22:20                                 ` Vivek Goyal
2011-08-26  1:56                                 ` Wu Fengguang
2011-08-26  1:56                                   ` Wu Fengguang
2011-08-26  8:56                                   ` Peter Zijlstra
2011-08-26  8:56                                     ` Peter Zijlstra
2011-08-26  9:53                                     ` Wu Fengguang
2011-08-26  9:53                                       ` Wu Fengguang
2011-08-29 13:12                             ` Peter Zijlstra
2011-08-29 13:12                               ` Peter Zijlstra
2011-08-29 13:37                               ` Wu Fengguang
2011-08-29 13:37                                 ` Wu Fengguang
2011-09-02 12:16                                 ` Peter Zijlstra
2011-09-02 12:16                                   ` Peter Zijlstra
2011-09-06 12:40                                 ` Peter Zijlstra
2011-09-06 12:40                                   ` Peter Zijlstra
2011-08-24 15:57                       ` Peter Zijlstra
2011-08-24 15:57                         ` Peter Zijlstra
2011-08-24 15:57                         ` Peter Zijlstra
2011-08-25  5:30                         ` Wu Fengguang
2011-08-25  5:30                           ` Wu Fengguang
2011-08-23 14:36                     ` Vivek Goyal
2011-08-23 14:36                       ` Vivek Goyal
2011-08-09  2:08   ` Vivek Goyal
2011-08-09  2:08     ` Vivek Goyal
2011-08-16  8:59     ` Wu Fengguang
2011-08-16  8:59       ` Wu Fengguang
2011-08-06  8:44 ` [PATCH 3/5] writeback: dirty rate control Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-09 14:54   ` Vivek Goyal
2011-08-09 14:54     ` Vivek Goyal
2011-08-11  3:42     ` Wu Fengguang
2011-08-11  3:42       ` Wu Fengguang
2011-08-09 14:57   ` Peter Zijlstra
2011-08-09 14:57     ` Peter Zijlstra
2011-08-09 14:57     ` Peter Zijlstra
2011-08-10 11:07     ` Wu Fengguang
2011-08-10 11:07       ` Wu Fengguang
2011-08-10 16:17       ` Peter Zijlstra
2011-08-10 16:17         ` Peter Zijlstra
2011-08-10 16:17         ` Peter Zijlstra
2011-08-15 14:08         ` Wu Fengguang
2011-08-15 14:08           ` Wu Fengguang
2011-08-09 15:50   ` Vivek Goyal
2011-08-09 15:50     ` Vivek Goyal
2011-08-09 16:16     ` Peter Zijlstra
2011-08-09 16:16       ` Peter Zijlstra
2011-08-09 16:16       ` Peter Zijlstra
2011-08-09 16:19       ` Peter Zijlstra
2011-08-09 16:19         ` Peter Zijlstra
2011-08-09 16:19         ` Peter Zijlstra
2011-08-10 14:07         ` Wu Fengguang
2011-08-10 14:07           ` Wu Fengguang
2011-08-10 14:00       ` Wu Fengguang
2011-08-10 14:00         ` Wu Fengguang
2011-08-10 17:10         ` Peter Zijlstra
2011-08-10 17:10           ` Peter Zijlstra
2011-08-15 14:11           ` Wu Fengguang
2011-08-15 14:11             ` Wu Fengguang
2011-08-09 16:56   ` Peter Zijlstra
2011-08-09 16:56     ` Peter Zijlstra
2011-08-09 16:56     ` Peter Zijlstra
2011-08-10 14:10     ` Wu Fengguang
2011-08-09 17:02   ` Peter Zijlstra
2011-08-09 17:02     ` Peter Zijlstra
2011-08-09 17:02     ` Peter Zijlstra
2011-08-10 14:15     ` Wu Fengguang
2011-08-10 14:15       ` Wu Fengguang
2011-08-06  8:44 ` [PATCH 4/5] writeback: per task dirty rate limit Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-06 14:35   ` Andrea Righi
2011-08-06 14:35     ` Andrea Righi
2011-08-07  6:19     ` Wu Fengguang
2011-08-07  6:19       ` Wu Fengguang
2011-08-08 13:47   ` Peter Zijlstra
2011-08-08 13:47     ` Peter Zijlstra
2011-08-08 13:47     ` Peter Zijlstra
2011-08-08 14:21     ` Wu Fengguang
2011-08-08 14:21       ` Wu Fengguang
2011-08-08 23:32       ` Wu Fengguang
2011-08-08 23:32         ` Wu Fengguang
2011-08-08 14:23     ` Wu Fengguang
2011-08-08 14:23       ` Wu Fengguang
2011-08-08 14:26       ` Peter Zijlstra
2011-08-08 14:26         ` Peter Zijlstra
2011-08-08 14:26         ` Peter Zijlstra
2011-08-08 22:38         ` Wu Fengguang
2011-08-08 22:38           ` Wu Fengguang
2011-08-13 16:28       ` Andrea Righi
2011-08-13 16:28         ` Andrea Righi
2011-08-15 14:21         ` Wu Fengguang
2011-08-15 14:26           ` Andrea Righi
2011-08-15 14:26             ` Andrea Righi
2011-08-09 17:46   ` Vivek Goyal
2011-08-09 17:46     ` Vivek Goyal
2011-08-10  3:29     ` Wu Fengguang
2011-08-10  3:29       ` Wu Fengguang
2011-08-10 18:18       ` Vivek Goyal
2011-08-10 18:18         ` Vivek Goyal
2011-08-11  0:55         ` Wu Fengguang
2011-08-11  0:55           ` Wu Fengguang
2011-08-09 18:35   ` Peter Zijlstra
2011-08-09 18:35     ` Peter Zijlstra
2011-08-09 18:35     ` Peter Zijlstra
2011-08-10  3:40     ` Wu Fengguang
2011-08-10  3:40       ` Wu Fengguang
2011-08-10 10:25       ` Peter Zijlstra
2011-08-10 10:25         ` Peter Zijlstra
2011-08-10 10:25         ` Peter Zijlstra
2011-08-10 11:13         ` Wu Fengguang
2011-08-10 11:13           ` Wu Fengguang
2011-08-06  8:44 ` [PATCH 5/5] writeback: IO-less balance_dirty_pages() Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-06  8:44   ` Wu Fengguang
2011-08-06 14:48   ` Andrea Righi
2011-08-06 14:48     ` Andrea Righi
2011-08-06 14:48     ` Andrea Righi
2011-08-07  6:44     ` Wu Fengguang
2011-08-07  6:44       ` Wu Fengguang
2011-08-07  6:44       ` Wu Fengguang
2011-08-06 16:46   ` Andrea Righi
2011-08-06 16:46     ` Andrea Righi
2011-08-07  7:18     ` Wu Fengguang
2011-08-07  9:50       ` Andrea Righi
2011-08-07  9:50         ` Andrea Righi
2011-08-09 18:15   ` Vivek Goyal
2011-08-09 18:15     ` Vivek Goyal
2011-08-09 18:41     ` Peter Zijlstra
2011-08-09 18:41       ` Peter Zijlstra
2011-08-09 18:41       ` Peter Zijlstra
2011-08-10  3:22       ` Wu Fengguang
2011-08-10  3:22         ` Wu Fengguang
2011-08-10  3:26     ` Wu Fengguang
2011-08-10  3:26       ` Wu Fengguang
2011-08-09 19:16   ` Vivek Goyal
2011-08-09 19:16     ` Vivek Goyal
2011-08-10  4:33     ` Wu Fengguang
2011-08-09  2:01 ` [PATCH 0/5] IO-less dirty throttling v8 Vivek Goyal
2011-08-09  2:01   ` Vivek Goyal
2011-08-09  5:55   ` Dave Chinner
2011-08-09  5:55     ` Dave Chinner
2011-08-09 14:04     ` Vivek Goyal
2011-08-09 14:04       ` Vivek Goyal
2011-08-10  7:41       ` Greg Thelen
2011-08-10  7:41         ` Greg Thelen
2011-08-10  7:41         ` Greg Thelen
2011-08-10 18:40         ` Vivek Goyal
2011-08-10 18:40           ` Vivek Goyal
2011-08-10 18:40           ` Vivek Goyal
2011-08-11  3:21   ` Wu Fengguang
2011-08-11  3:21     ` Wu Fengguang
2011-08-11 20:42     ` Vivek Goyal
2011-08-11 20:42       ` Vivek Goyal
2011-08-11 21:00       ` Vivek Goyal
2011-08-11 21:00         ` Vivek Goyal
2011-08-16  2:20 [PATCH 0/5] IO-less dirty throttling v9 Wu Fengguang
2011-08-16  2:20 ` [PATCH 2/5] writeback: dirty position control Wu Fengguang
2011-08-16  2:20   ` Wu Fengguang
2011-08-16  2:20   ` Wu Fengguang
2011-08-16 19:41   ` Jan Kara
2011-08-16 19:41     ` Jan Kara
2011-08-17 13:23     ` Wu Fengguang
2011-08-17 13:49       ` Wu Fengguang
2011-08-17 13:49         ` Wu Fengguang
2011-08-17 20:24       ` Jan Kara
2011-08-17 20:24         ` Jan Kara
2011-08-18  4:18         ` Wu Fengguang
2011-08-18  4:18           ` Wu Fengguang
2011-08-18  4:41           ` Wu Fengguang
2011-08-18  4:41             ` Wu Fengguang
2011-08-18 19:16           ` Jan Kara
2011-08-18 19:16             ` Jan Kara
2011-08-24  3:16         ` Wu Fengguang
2011-08-24  3:16           ` Wu Fengguang
2011-08-19  2:53   ` Vivek Goyal
2011-08-19  2:53     ` Vivek Goyal
2011-08-19  3:25     ` Wu Fengguang
2011-08-19  3:25       ` Wu Fengguang
2011-08-17  6:40 ` David Horner
2011-08-17 12:03   ` Jan Kara
2011-08-17 12:35     ` Wu Fengguang
```

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