[PATCH 09/17] writeback: bdi write bandwidth estimation
From: Wu Fengguang
Date: Sun Sep 12 2010 - 11:55:56 EST
The estimation value will start from 100MB/s and adapt to the real
bandwidth in seconds. It's pretty accurate for common filesystems.
As the first use case, it replaces the static 100MB/s value used for
'bw' calculation in balance_dirty_pages().
The overheads won't be high because the bdi bandwidth udpate only occurs
in >10ms intervals.
Initially it's only estimated in balance_dirty_pages() because this is
the most reliable place to get reasonable large bandwidth -- the bdi is
normally fully utilized when bdi_thresh is reached.
Then Shaohua recommends to also do it in the flusher thread, to keep the
value updated when there are only periodic/background writeback and no
tasks throttled.
The estimation cannot be done purely in the flusher thread because it's
not sufficient for NFS. NFS writeback won't block at get_request_wait(),
so tend to complete quickly. Another problem is, slow devices may take
dozens of seconds to write the initial 64MB chunk (write_bandwidth
starts with 100MB/s, this translates to 64MB nr_to_write). So it may
take more than 1 minute to adapt to the smallish bandwidth if the
bandwidth is only updated in the flusher thread.
CC: Li Shaohua <shaohua.li@xxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
fs/fs-writeback.c | 4 ++++
include/linux/backing-dev.h | 1 +
include/linux/writeback.h | 3 +++
mm/backing-dev.c | 1 +
mm/page-writeback.c | 33 ++++++++++++++++++++++++++++++++-
5 files changed, 41 insertions(+), 1 deletion(-)
--- linux-next.orig/include/linux/backing-dev.h 2010-09-09 16:02:43.000000000 +0800
+++ linux-next/include/linux/backing-dev.h 2010-09-09 16:02:45.000000000 +0800
@@ -76,6 +76,7 @@ struct backing_dev_info {
struct prop_local_percpu completions;
int dirty_exceeded;
+ int write_bandwidth;
unsigned int min_ratio;
unsigned int max_ratio, max_prop_frac;
--- linux-next.orig/mm/backing-dev.c 2010-09-09 16:02:43.000000000 +0800
+++ linux-next/mm/backing-dev.c 2010-09-09 16:02:45.000000000 +0800
@@ -658,6 +658,7 @@ int bdi_init(struct backing_dev_info *bd
goto err;
}
+ bdi->write_bandwidth = 100 << 20;
bdi->dirty_exceeded = 0;
err = prop_local_init_percpu(&bdi->completions);
--- linux-next.orig/fs/fs-writeback.c 2010-09-09 14:13:21.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2010-09-09 16:02:46.000000000 +0800
@@ -603,6 +603,8 @@ static long wb_writeback(struct bdi_writ
.range_cyclic = work->range_cyclic,
};
unsigned long oldest_jif;
+ unsigned long bw_time;
+ s64 bw_written = 0;
long wrote = 0;
struct inode *inode;
@@ -616,6 +618,7 @@ static long wb_writeback(struct bdi_writ
wbc.range_end = LLONG_MAX;
}
+ bdi_update_write_bandwidth(wb->bdi, &bw_time, &bw_written);
wbc.wb_start = jiffies; /* livelock avoidance */
for (;;) {
/*
@@ -641,6 +644,7 @@ static long wb_writeback(struct bdi_writ
else
writeback_inodes_wb(wb, &wbc);
trace_wbc_writeback_written(&wbc, wb->bdi);
+ bdi_update_write_bandwidth(wb->bdi, &bw_time, &bw_written);
work->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
--- linux-next.orig/mm/page-writeback.c 2010-09-09 16:02:43.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-09-09 16:04:23.000000000 +0800
@@ -449,6 +449,32 @@ unsigned long bdi_dirty_limit(struct bac
return bdi_dirty;
}
+void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
+ unsigned long *bw_time,
+ s64 *bw_written)
+{
+ unsigned long pages;
+ unsigned long time;
+ unsigned long bw;
+ unsigned long w;
+
+ if (*bw_written == 0)
+ goto start_over;
+
+ time = jiffies - *bw_time;
+ if (time < HZ/100)
+ return;
+
+ pages = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]) - *bw_written;
+ bw = HZ * PAGE_CACHE_SIZE * pages / time;
+ w = clamp_t(unsigned long, time / (HZ/100), 1, 128);
+
+ bdi->write_bandwidth = (bdi->write_bandwidth * (1024-w) + bw * w) >> 10;
+start_over:
+ *bw_written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
+ *bw_time = jiffies;
+}
+
/*
* balance_dirty_pages() must be called by processes which are generating dirty
* data. It looks at the number of dirty pages in the machine and will force
@@ -471,6 +497,8 @@ static void balance_dirty_pages(struct a
bool dirty_exceeded = false;
struct backing_dev_info *bdi = mapping->backing_dev_info;
long numerator, denominator;
+ unsigned long bw_time;
+ s64 bw_written = 0;
for (;;) {
/*
@@ -536,10 +564,12 @@ static void balance_dirty_pages(struct a
bdi_thresh - bdi_thresh / DIRTY_SOFT_THROTTLE_RATIO)
goto check_exceeded;
+ bdi_update_write_bandwidth(bdi, &bw_time, &bw_written);
+
gap = bdi_thresh > (bdi_nr_reclaimable + bdi_nr_writeback) ?
bdi_thresh - (bdi_nr_reclaimable + bdi_nr_writeback) : 0;
- bw = (100 << 20) * gap /
+ bw = bdi->write_bandwidth * gap /
(bdi_thresh / DIRTY_SOFT_THROTTLE_RATIO + 1);
pause = HZ * (pages_dirtied << PAGE_CACHE_SHIFT) / (bw + 1);
@@ -562,6 +592,7 @@ static void balance_dirty_pages(struct a
if (signal_pending(current))
break;
+ bdi_update_write_bandwidth(bdi, &bw_time, &bw_written);
check_exceeded:
/*
* The bdi thresh is somehow "soft" limit derived from the
--- linux-next.orig/include/linux/writeback.h 2010-09-09 15:51:38.000000000 +0800
+++ linux-next/include/linux/writeback.h 2010-09-09 16:02:46.000000000 +0800
@@ -136,6 +136,9 @@ int dirty_writeback_centisecs_handler(st
void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty);
unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
unsigned long dirty);
+void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
+ unsigned long *bw_time,
+ s64 *bw_written);
void page_writeback_init(void);
void balance_dirty_pages_ratelimited_nr(struct address_space *mapping,
--
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