From f3109a51f8dc88e8a94f620240b7474b91bed37a Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Thu, 24 May 2012 18:59:10 +0200 Subject: lib: Proportions with flexible period Implement code computing proportions of events of different type (like code in lib/proportions.c) but allowing periods to have different lengths. This allows us to have aging periods of fixed wallclock time which gives better proportion estimates given the hugely varying throughput of different devices - previous measuring of aging period by number of events has the problem that a reasonable period length for a system with low-end USB stick is not a reasonable period length for a system with high-end storage array resulting either in too slow proportion updates or too fluctuating proportion updates. Acked-by: Peter Zijlstra Signed-off-by: Jan Kara Signed-off-by: Fengguang Wu diff --git a/include/linux/flex_proportions.h b/include/linux/flex_proportions.h new file mode 100644 index 0000000..4ebc49f --- /dev/null +++ b/include/linux/flex_proportions.h @@ -0,0 +1,101 @@ +/* + * Floating proportions with flexible aging period + * + * Copyright (C) 2011, SUSE, Jan Kara + */ + +#ifndef _LINUX_FLEX_PROPORTIONS_H +#define _LINUX_FLEX_PROPORTIONS_H + +#include +#include +#include + +/* + * When maximum proportion of some event type is specified, this is the + * precision with which we allow limitting. Note that this creates an upper + * bound on the number of events per period like + * ULLONG_MAX >> FPROP_FRAC_SHIFT. + */ +#define FPROP_FRAC_SHIFT 10 +#define FPROP_FRAC_BASE (1UL << FPROP_FRAC_SHIFT) + +/* + * ---- Global proportion definitions ---- + */ +struct fprop_global { + /* Number of events in the current period */ + struct percpu_counter events; + /* Current period */ + unsigned int period; + /* Synchronization with period transitions */ + seqcount_t sequence; +}; + +int fprop_global_init(struct fprop_global *p); +void fprop_global_destroy(struct fprop_global *p); +bool fprop_new_period(struct fprop_global *p, int periods); + +/* + * ---- SINGLE ---- + */ +struct fprop_local_single { + /* the local events counter */ + unsigned long events; + /* Period in which we last updated events */ + unsigned int period; + raw_spinlock_t lock; /* Protect period and numerator */ +}; + +#define INIT_FPROP_LOCAL_SINGLE(name) \ +{ .lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock), \ +} + +int fprop_local_init_single(struct fprop_local_single *pl); +void fprop_local_destroy_single(struct fprop_local_single *pl); +void __fprop_inc_single(struct fprop_global *p, struct fprop_local_single *pl); +void fprop_fraction_single(struct fprop_global *p, + struct fprop_local_single *pl, unsigned long *numerator, + unsigned long *denominator); + +static inline +void fprop_inc_single(struct fprop_global *p, struct fprop_local_single *pl) +{ + unsigned long flags; + + local_irq_save(flags); + __fprop_inc_single(p, pl); + local_irq_restore(flags); +} + +/* + * ---- PERCPU ---- + */ +struct fprop_local_percpu { + /* the local events counter */ + struct percpu_counter events; + /* Period in which we last updated events */ + unsigned int period; + raw_spinlock_t lock; /* Protect period and numerator */ +}; + +int fprop_local_init_percpu(struct fprop_local_percpu *pl); +void fprop_local_destroy_percpu(struct fprop_local_percpu *pl); +void __fprop_inc_percpu(struct fprop_global *p, struct fprop_local_percpu *pl); +void __fprop_inc_percpu_max(struct fprop_global *p, struct fprop_local_percpu *pl, + int max_frac); +void fprop_fraction_percpu(struct fprop_global *p, + struct fprop_local_percpu *pl, unsigned long *numerator, + unsigned long *denominator); + +static inline +void fprop_inc_percpu(struct fprop_global *p, struct fprop_local_percpu *pl) +{ + unsigned long flags; + + local_irq_save(flags); + __fprop_inc_percpu(p, pl); + local_irq_restore(flags); +} + +#endif diff --git a/lib/Makefile b/lib/Makefile index 8c31a0c..ee83737 100644 --- a/lib/Makefile +++ b/lib/Makefile @@ -11,7 +11,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \ rbtree.o radix-tree.o dump_stack.o timerqueue.o\ idr.o int_sqrt.o extable.o prio_tree.o \ sha1.o md5.o irq_regs.o reciprocal_div.o argv_split.o \ - proportions.o prio_heap.o ratelimit.o show_mem.o \ + proportions.o flex_proportions.o prio_heap.o ratelimit.o show_mem.o \ is_single_threaded.o plist.o decompress.o lib-$(CONFIG_MMU) += ioremap.o diff --git a/lib/flex_proportions.c b/lib/flex_proportions.c new file mode 100644 index 0000000..e02a388 --- /dev/null +++ b/lib/flex_proportions.c @@ -0,0 +1,266 @@ +/* + * Floating proportions with flexible aging period + * + * Copyright (C) 2011, SUSE, Jan Kara + * + * The goal of this code is: Given different types of event, measure proportion + * of each type of event over time. The proportions are measured with + * exponentially decaying history to give smooth transitions. A formula + * expressing proportion of event of type 'j' is: + * + * p_{j} = (\Sum_{i>=0} x_{i,j}/2^{i+1})/(\Sum_{i>=0} x_i/2^{i+1}) + * + * Where x_{i,j} is j's number of events in i-th last time period and x_i is + * total number of events in i-th last time period. + * + * Note that p_{j}'s are normalised, i.e. + * + * \Sum_{j} p_{j} = 1, + * + * This formula can be straightforwardly computed by maintaing denominator + * (let's call it 'd') and for each event type its numerator (let's call it + * 'n_j'). When an event of type 'j' happens, we simply need to do: + * n_j++; d++; + * + * When a new period is declared, we could do: + * d /= 2 + * for each j + * n_j /= 2 + * + * To avoid iteration over all event types, we instead shift numerator of event + * j lazily when someone asks for a proportion of event j or when event j + * occurs. This can bit trivially implemented by remembering last period in + * which something happened with proportion of type j. + */ +#include + +int fprop_global_init(struct fprop_global *p) +{ + int err; + + p->period = 0; + /* Use 1 to avoid dealing with periods with 0 events... */ + err = percpu_counter_init(&p->events, 1); + if (err) + return err; + seqcount_init(&p->sequence); + return 0; +} + +void fprop_global_destroy(struct fprop_global *p) +{ + percpu_counter_destroy(&p->events); +} + +/* + * Declare @periods new periods. It is upto the caller to make sure period + * transitions cannot happen in parallel. + * + * The function returns true if the proportions are still defined and false + * if aging zeroed out all events. This can be used to detect whether declaring + * further periods has any effect. + */ +bool fprop_new_period(struct fprop_global *p, int periods) +{ + u64 events = percpu_counter_sum(&p->events); + + /* + * Don't do anything if there are no events. + */ + if (events <= 1) + return false; + write_seqcount_begin(&p->sequence); + if (periods < 64) + events -= events >> periods; + /* Use addition to avoid losing events happening between sum and set */ + percpu_counter_add(&p->events, -events); + p->period += periods; + write_seqcount_end(&p->sequence); + + return true; +} + +/* + * ---- SINGLE ---- + */ + +int fprop_local_init_single(struct fprop_local_single *pl) +{ + pl->events = 0; + pl->period = 0; + raw_spin_lock_init(&pl->lock); + return 0; +} + +void fprop_local_destroy_single(struct fprop_local_single *pl) +{ +} + +static void fprop_reflect_period_single(struct fprop_global *p, + struct fprop_local_single *pl) +{ + unsigned int period = p->period; + unsigned long flags; + + /* Fast path - period didn't change */ + if (pl->period == period) + return; + raw_spin_lock_irqsave(&pl->lock, flags); + /* Someone updated pl->period while we were spinning? */ + if (pl->period >= period) { + raw_spin_unlock_irqrestore(&pl->lock, flags); + return; + } + /* Aging zeroed our fraction? */ + if (period - pl->period < BITS_PER_LONG) + pl->events >>= period - pl->period; + else + pl->events = 0; + pl->period = period; + raw_spin_unlock_irqrestore(&pl->lock, flags); +} + +/* Event of type pl happened */ +void __fprop_inc_single(struct fprop_global *p, struct fprop_local_single *pl) +{ + fprop_reflect_period_single(p, pl); + pl->events++; + percpu_counter_add(&p->events, 1); +} + +/* Return fraction of events of type pl */ +void fprop_fraction_single(struct fprop_global *p, + struct fprop_local_single *pl, + unsigned long *numerator, unsigned long *denominator) +{ + unsigned int seq; + s64 num, den; + + do { + seq = read_seqcount_begin(&p->sequence); + fprop_reflect_period_single(p, pl); + num = pl->events; + den = percpu_counter_read_positive(&p->events); + } while (read_seqcount_retry(&p->sequence, seq)); + + /* + * Make fraction <= 1 and denominator > 0 even in presence of percpu + * counter errors + */ + if (den <= num) { + if (num) + den = num; + else + den = 1; + } + *denominator = den; + *numerator = num; +} + +/* + * ---- PERCPU ---- + */ +#define PROP_BATCH (8*(1+ilog2(nr_cpu_ids))) + +int fprop_local_init_percpu(struct fprop_local_percpu *pl) +{ + int err; + + err = percpu_counter_init(&pl->events, 0); + if (err) + return err; + pl->period = 0; + raw_spin_lock_init(&pl->lock); + return 0; +} + +void fprop_local_destroy_percpu(struct fprop_local_percpu *pl) +{ + percpu_counter_destroy(&pl->events); +} + +static void fprop_reflect_period_percpu(struct fprop_global *p, + struct fprop_local_percpu *pl) +{ + unsigned int period = p->period; + unsigned long flags; + + /* Fast path - period didn't change */ + if (pl->period == period) + return; + raw_spin_lock_irqsave(&pl->lock, flags); + /* Someone updated pl->period while we were spinning? */ + if (pl->period >= period) { + raw_spin_unlock_irqrestore(&pl->lock, flags); + return; + } + /* Aging zeroed our fraction? */ + if (period - pl->period < BITS_PER_LONG) { + s64 val = percpu_counter_read(&pl->events); + + if (val < (nr_cpu_ids * PROP_BATCH)) + val = percpu_counter_sum(&pl->events); + + __percpu_counter_add(&pl->events, + -val + (val >> (period-pl->period)), PROP_BATCH); + } else + percpu_counter_set(&pl->events, 0); + pl->period = period; + raw_spin_unlock_irqrestore(&pl->lock, flags); +} + +/* Event of type pl happened */ +void __fprop_inc_percpu(struct fprop_global *p, struct fprop_local_percpu *pl) +{ + fprop_reflect_period_percpu(p, pl); + __percpu_counter_add(&pl->events, 1, PROP_BATCH); + percpu_counter_add(&p->events, 1); +} + +void fprop_fraction_percpu(struct fprop_global *p, + struct fprop_local_percpu *pl, + unsigned long *numerator, unsigned long *denominator) +{ + unsigned int seq; + s64 num, den; + + do { + seq = read_seqcount_begin(&p->sequence); + fprop_reflect_period_percpu(p, pl); + num = percpu_counter_read_positive(&pl->events); + den = percpu_counter_read_positive(&p->events); + } while (read_seqcount_retry(&p->sequence, seq)); + + /* + * Make fraction <= 1 and denominator > 0 even in presence of percpu + * counter errors + */ + if (den <= num) { + if (num) + den = num; + else + den = 1; + } + *denominator = den; + *numerator = num; +} + +/* + * Like __fprop_inc_percpu() except that event is counted only if the given + * type has fraction smaller than @max_frac/FPROP_FRAC_BASE + */ +void __fprop_inc_percpu_max(struct fprop_global *p, + struct fprop_local_percpu *pl, int max_frac) +{ + if (unlikely(max_frac < FPROP_FRAC_BASE)) { + unsigned long numerator, denominator; + + fprop_fraction_percpu(p, pl, &numerator, &denominator); + if (numerator > + (((u64)denominator) * max_frac) >> FPROP_FRAC_SHIFT) + return; + } else + fprop_reflect_period_percpu(p, pl); + __percpu_counter_add(&pl->events, 1, PROP_BATCH); + percpu_counter_add(&p->events, 1); +} -- cgit v0.10.2 From e78d4833c03e28205b3d983f0c4e586ee34785fd Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Fri, 1 Jun 2012 00:42:06 +0200 Subject: lib: Fix possible deadlock in flexible proportion code When percpu counter function in fprop_new_period() is interrupted by an interrupt while holding counter lock, it can cause deadlock when the interrupt wants to take the lock as well. Fix the problem by disabling interrupts when calling percpu counter functions. Signed-off-by: Jan Kara Signed-off-by: Fengguang Wu diff --git a/lib/flex_proportions.c b/lib/flex_proportions.c index e02a388..c785554 100644 --- a/lib/flex_proportions.c +++ b/lib/flex_proportions.c @@ -62,13 +62,18 @@ void fprop_global_destroy(struct fprop_global *p) */ bool fprop_new_period(struct fprop_global *p, int periods) { - u64 events = percpu_counter_sum(&p->events); + u64 events; + unsigned long flags; + local_irq_save(flags); + events = percpu_counter_sum(&p->events); /* * Don't do anything if there are no events. */ - if (events <= 1) + if (events <= 1) { + local_irq_restore(flags); return false; + } write_seqcount_begin(&p->sequence); if (periods < 64) events -= events >> periods; @@ -76,6 +81,7 @@ bool fprop_new_period(struct fprop_global *p, int periods) percpu_counter_add(&p->events, -events); p->period += periods; write_seqcount_end(&p->sequence); + local_irq_restore(flags); return true; } -- cgit v0.10.2 From eb608e3a344b3af21300360fcf868f8b4e808a8e Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Thu, 24 May 2012 18:59:11 +0200 Subject: block: Convert BDI proportion calculations to flexible proportions Convert calculations of proportion of writeback each bdi does to new flexible proportion code. That allows us to use aging period of fixed wallclock time which gives better proportion estimates given the hugely varying throughput of different devices. Acked-by: Peter Zijlstra Signed-off-by: Jan Kara Signed-off-by: Fengguang Wu diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h index b1038bd..489de62 100644 --- a/include/linux/backing-dev.h +++ b/include/linux/backing-dev.h @@ -10,7 +10,7 @@ #include #include -#include +#include #include #include #include @@ -89,7 +89,7 @@ struct backing_dev_info { unsigned long dirty_ratelimit; unsigned long balanced_dirty_ratelimit; - struct prop_local_percpu completions; + struct fprop_local_percpu completions; int dirty_exceeded; unsigned int min_ratio; diff --git a/mm/backing-dev.c b/mm/backing-dev.c index dd8e2aa..3387aea 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -677,7 +677,7 @@ int bdi_init(struct backing_dev_info *bdi) bdi->min_ratio = 0; bdi->max_ratio = 100; - bdi->max_prop_frac = PROP_FRAC_BASE; + bdi->max_prop_frac = FPROP_FRAC_BASE; spin_lock_init(&bdi->wb_lock); INIT_LIST_HEAD(&bdi->bdi_list); INIT_LIST_HEAD(&bdi->work_list); @@ -700,7 +700,7 @@ int bdi_init(struct backing_dev_info *bdi) bdi->write_bandwidth = INIT_BW; bdi->avg_write_bandwidth = INIT_BW; - err = prop_local_init_percpu(&bdi->completions); + err = fprop_local_init_percpu(&bdi->completions); if (err) { err: @@ -744,7 +744,7 @@ void bdi_destroy(struct backing_dev_info *bdi) for (i = 0; i < NR_BDI_STAT_ITEMS; i++) percpu_counter_destroy(&bdi->bdi_stat[i]); - prop_local_destroy_percpu(&bdi->completions); + fprop_local_destroy_percpu(&bdi->completions); } EXPORT_SYMBOL(bdi_destroy); diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 93d8d2f..ec14419 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -34,6 +34,7 @@ #include #include /* __set_page_dirty_buffers */ #include +#include #include /* @@ -135,7 +136,20 @@ unsigned long global_dirty_limit; * measured in page writeback completions. * */ -static struct prop_descriptor vm_completions; +static struct fprop_global writeout_completions; + +static void writeout_period(unsigned long t); +/* Timer for aging of writeout_completions */ +static struct timer_list writeout_period_timer = + TIMER_DEFERRED_INITIALIZER(writeout_period, 0, 0); +static unsigned long writeout_period_time = 0; + +/* + * Length of period for aging writeout fractions of bdis. This is an + * arbitrarily chosen number. The longer the period, the slower fractions will + * reflect changes in current writeout rate. + */ +#define VM_COMPLETIONS_PERIOD_LEN (3*HZ) /* * Work out the current dirty-memory clamping and background writeout @@ -322,34 +336,6 @@ bool zone_dirty_ok(struct zone *zone) zone_page_state(zone, NR_WRITEBACK) <= limit; } -/* - * couple the period to the dirty_ratio: - * - * period/2 ~ roundup_pow_of_two(dirty limit) - */ -static int calc_period_shift(void) -{ - unsigned long dirty_total; - - if (vm_dirty_bytes) - dirty_total = vm_dirty_bytes / PAGE_SIZE; - else - dirty_total = (vm_dirty_ratio * global_dirtyable_memory()) / - 100; - return 2 + ilog2(dirty_total - 1); -} - -/* - * update the period when the dirty threshold changes. - */ -static void update_completion_period(void) -{ - int shift = calc_period_shift(); - prop_change_shift(&vm_completions, shift); - - writeback_set_ratelimit(); -} - int dirty_background_ratio_handler(struct ctl_table *table, int write, void __user *buffer, size_t *lenp, loff_t *ppos) @@ -383,7 +369,7 @@ int dirty_ratio_handler(struct ctl_table *table, int write, ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos); if (ret == 0 && write && vm_dirty_ratio != old_ratio) { - update_completion_period(); + writeback_set_ratelimit(); vm_dirty_bytes = 0; } return ret; @@ -398,12 +384,21 @@ int dirty_bytes_handler(struct ctl_table *table, int write, ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos); if (ret == 0 && write && vm_dirty_bytes != old_bytes) { - update_completion_period(); + writeback_set_ratelimit(); vm_dirty_ratio = 0; } return ret; } +static unsigned long wp_next_time(unsigned long cur_time) +{ + cur_time += VM_COMPLETIONS_PERIOD_LEN; + /* 0 has a special meaning... */ + if (!cur_time) + return 1; + return cur_time; +} + /* * Increment the BDI's writeout completion count and the global writeout * completion count. Called from test_clear_page_writeback(). @@ -411,8 +406,19 @@ int dirty_bytes_handler(struct ctl_table *table, int write, static inline void __bdi_writeout_inc(struct backing_dev_info *bdi) { __inc_bdi_stat(bdi, BDI_WRITTEN); - __prop_inc_percpu_max(&vm_completions, &bdi->completions, - bdi->max_prop_frac); + __fprop_inc_percpu_max(&writeout_completions, &bdi->completions, + bdi->max_prop_frac); + /* First event after period switching was turned off? */ + if (!unlikely(writeout_period_time)) { + /* + * We can race with other __bdi_writeout_inc calls here but + * it does not cause any harm since the resulting time when + * timer will fire and what is in writeout_period_time will be + * roughly the same. + */ + writeout_period_time = wp_next_time(jiffies); + mod_timer(&writeout_period_timer, writeout_period_time); + } } void bdi_writeout_inc(struct backing_dev_info *bdi) @@ -431,11 +437,33 @@ EXPORT_SYMBOL_GPL(bdi_writeout_inc); static void bdi_writeout_fraction(struct backing_dev_info *bdi, long *numerator, long *denominator) { - prop_fraction_percpu(&vm_completions, &bdi->completions, + fprop_fraction_percpu(&writeout_completions, &bdi->completions, numerator, denominator); } /* + * On idle system, we can be called long after we scheduled because we use + * deferred timers so count with missed periods. + */ +static void writeout_period(unsigned long t) +{ + int miss_periods = (jiffies - writeout_period_time) / + VM_COMPLETIONS_PERIOD_LEN; + + if (fprop_new_period(&writeout_completions, miss_periods + 1)) { + writeout_period_time = wp_next_time(writeout_period_time + + miss_periods * VM_COMPLETIONS_PERIOD_LEN); + mod_timer(&writeout_period_timer, writeout_period_time); + } else { + /* + * Aging has zeroed all fractions. Stop wasting CPU on period + * updates. + */ + writeout_period_time = 0; + } +} + +/* * bdi_min_ratio keeps the sum of the minimum dirty shares of all * registered backing devices, which, for obvious reasons, can not * exceed 100%. @@ -475,7 +503,7 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio) ret = -EINVAL; } else { bdi->max_ratio = max_ratio; - bdi->max_prop_frac = (PROP_FRAC_BASE * max_ratio) / 100; + bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100; } spin_unlock_bh(&bdi_lock); @@ -1606,13 +1634,10 @@ static struct notifier_block __cpuinitdata ratelimit_nb = { */ void __init page_writeback_init(void) { - int shift; - writeback_set_ratelimit(); register_cpu_notifier(&ratelimit_nb); - shift = calc_period_shift(); - prop_descriptor_init(&vm_completions, shift); + fprop_global_init(&writeout_completions); } /** -- cgit v0.10.2 From 331cbdeedeb2f4ef01ccb761513708af0fe77098 Mon Sep 17 00:00:00 2001 From: Wanpeng Li Date: Sat, 9 Jun 2012 11:10:55 +0800 Subject: writeback: Fix some comment errors Signed-off-by: Wanpeng Li Signed-off-by: Fengguang Wu diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 41a3ccf..0b2c87e 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -628,8 +628,8 @@ static long writeback_sb_inodes(struct super_block *sb, } /* - * Don't bother with new inodes or inodes beeing freed, first - * kind does not need peridic writeout yet, and for the latter + * Don't bother with new inodes or inodes being freed, first + * kind does not need periodic writeout yet, and for the latter * kind writeout is handled by the freer. */ spin_lock(&inode->i_lock); diff --git a/fs/super.c b/fs/super.c index cf00177..3d65443 100644 --- a/fs/super.c +++ b/fs/super.c @@ -318,7 +318,7 @@ static int grab_super(struct super_block *s) __releases(sb_lock) /* * grab_super_passive - acquire a passive reference - * @s: reference we are trying to grab + * @sb: reference we are trying to grab * * Tries to acquire a passive reference. This is used in places where we * cannot take an active reference but we need to ensure that the diff --git a/fs/sync.c b/fs/sync.c index 11e3d1c..1830704 100644 --- a/fs/sync.c +++ b/fs/sync.c @@ -92,7 +92,7 @@ static void sync_filesystems(int wait) } /* - * sync everything. Start out by waking pdflush, because that writes back + * sync everything. Start out by waking flusher, because that writes back * all queues in parallel. */ SYSCALL_DEFINE0(sync) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index ec14419..e5363f3 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -946,7 +946,7 @@ static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi, * bdi->dirty_ratelimit = balanced_dirty_ratelimit; * * However to get a more stable dirty_ratelimit, the below elaborated - * code makes use of task_ratelimit to filter out sigular points and + * code makes use of task_ratelimit to filter out singular points and * limit the step size. * * The below code essentially only uses the relative value of @@ -969,7 +969,7 @@ static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi, * feel and care are stable dirty rate and small position error. * * |task_ratelimit - dirty_ratelimit| is used to limit the step size - * and filter out the sigular points of balanced_dirty_ratelimit. Which + * and filter out the singular points of balanced_dirty_ratelimit. Which * keeps jumping around randomly and can even leap far away at times * due to the small 200ms estimation period of dirty_rate (we want to * keep that period small to reduce time lags). -- cgit v0.10.2