On Mon, 22 Feb 2016, Johannes Weiner wrote: > In machines with 140G of memory and enterprise flash storage, we have > seen read and write bursts routinely exceed the kswapd watermarks and > cause thundering herds in direct reclaim. Unfortunately, the only way > to tune kswapd aggressiveness is through adjusting min_free_kbytes - > the system's emergency reserves - which is entirely unrelated to the > system's latency requirements. In order to get kswapd to maintain a > 250M buffer of free memory, the emergency reserves need to be set to > 1G. That is a lot of memory wasted for no good reason. > > On the other hand, it's reasonable to assume that allocation bursts > and overall allocation concurrency scale with memory capacity, so it > makes sense to make kswapd aggressiveness a function of that as well. > > Change the kswapd watermark scale factor from the currently fixed 25% > of the tunable emergency reserve to a tunable 0.001% of memory. > Making this tunable independent of min_free_kbytes is great. I'm wondering how the choice of 0.001% was picked for default? One of my workstations currently has step sizes of about 0.0005% so this will be doubling the steps from min to low and low to high. I'm not objecting to that since it's definitely in the right direction (more free memory) but I wonder if it will make a difference for some users. > Beyond 1G of memory, this will produce bigger watermark steps than the > current formula in default settings. Ensure that the new formula never > chooses steps smaller than that, i.e. 25% of the emergency reserve. > > On a 140G machine, this raises the default watermark steps - the > distance between min and low, and low and high - from 16M to 143M. > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> > Acked-by: Mel Gorman <mgorman@xxxxxxx> > --- > Documentation/sysctl/vm.txt | 18 ++++++++++++++++++ > include/linux/mm.h | 1 + > include/linux/mmzone.h | 2 ++ > kernel/sysctl.c | 10 ++++++++++ > mm/page_alloc.c | 29 +++++++++++++++++++++++++++-- > 5 files changed, 58 insertions(+), 2 deletions(-) > > v2: Ensure 25% of emergency reserves as a minimum on small machines -Rik > > diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt > index 89a887c..b02d940 100644 > --- a/Documentation/sysctl/vm.txt > +++ b/Documentation/sysctl/vm.txt > @@ -803,6 +803,24 @@ performance impact. Reclaim code needs to take various locks to find freeable > directory and inode objects. With vfs_cache_pressure=1000, it will look for > ten times more freeable objects than there are. > > +============================================================= > + > +watermark_scale_factor: > + > +This factor controls the aggressiveness of kswapd. It defines the > +amount of memory left in a node/system before kswapd is woken up and > +how much memory needs to be free before kswapd goes back to sleep. > + > +The unit is in fractions of 10,000. The default value of 10 means the > +distances between watermarks are 0.001% of the available memory in the > +node/system. The maximum value is 1000, or 10% of memory. > + The effective maximum value can be different than the tunable, though, correct? It seems like you'd want to document why watermark_scale_factor and the actual watermarks in /proc/zoneinfo may be different on some systems. > +A high rate of threads entering direct reclaim (allocstall) or kswapd > +going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate > +that the number of free pages kswapd maintains for latency reasons is > +too small for the allocation bursts occurring in the system. This knob > +can then be used to tune kswapd aggressiveness accordingly. > + > ============================================================== > > zone_reclaim_mode: > diff --git a/include/linux/mm.h b/include/linux/mm.h > index a0ad7af..d330cbb 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1869,6 +1869,7 @@ extern void zone_pcp_reset(struct zone *zone); > > /* page_alloc.c */ > extern int min_free_kbytes; > +extern int watermark_scale_factor; > > /* nommu.c */ > extern atomic_long_t mmap_pages_allocated; > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 03cbdd9..85d6702 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -833,6 +833,8 @@ static inline int is_highmem(struct zone *zone) > struct ctl_table; > int min_free_kbytes_sysctl_handler(struct ctl_table *, int, > void __user *, size_t *, loff_t *); > +int watermark_scale_factor_sysctl_handler(struct ctl_table *, int, > + void __user *, size_t *, loff_t *); > extern int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES-1]; > int lowmem_reserve_ratio_sysctl_handler(struct ctl_table *, int, > void __user *, size_t *, loff_t *); > diff --git a/kernel/sysctl.c b/kernel/sysctl.c > index d479707..780769e 100644 > --- a/kernel/sysctl.c > +++ b/kernel/sysctl.c > @@ -126,6 +126,7 @@ static int __maybe_unused two = 2; > static int __maybe_unused four = 4; > static unsigned long one_ul = 1; > static int one_hundred = 100; > +static int one_thousand = 1000; > #ifdef CONFIG_PRINTK > static int ten_thousand = 10000; > #endif > @@ -1393,6 +1394,15 @@ static struct ctl_table vm_table[] = { > .extra1 = &zero, > }, > { > + .procname = "watermark_scale_factor", > + .data = &watermark_scale_factor, > + .maxlen = sizeof(watermark_scale_factor), > + .mode = 0644, > + .proc_handler = watermark_scale_factor_sysctl_handler, > + .extra1 = &one, > + .extra2 = &one_thousand, > + }, > + { > .procname = "percpu_pagelist_fraction", > .data = &percpu_pagelist_fraction, > .maxlen = sizeof(percpu_pagelist_fraction), > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 0c3eba3..0f457bb 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -249,6 +249,7 @@ compound_page_dtor * const compound_page_dtors[] = { > > int min_free_kbytes = 1024; > int user_min_free_kbytes = -1; > +int watermark_scale_factor = 10; > > static unsigned long __meminitdata nr_kernel_pages; > static unsigned long __meminitdata nr_all_pages; > @@ -6330,8 +6331,17 @@ static void __setup_per_zone_wmarks(void) > zone->watermark[WMARK_MIN] = tmp; > } > > - zone->watermark[WMARK_LOW] = min_wmark_pages(zone) + (tmp >> 2); > - zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp >> 1); > + /* > + * Set the kswapd watermarks distance according to the > + * scale factor in proportion to available memory, but > + * ensure a minimum size on small systems. > + */ > + tmp = max_t(u64, tmp >> 2, > + mult_frac(zone->managed_pages, > + watermark_scale_factor, 10000)); > + > + zone->watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp; > + zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2; > > __mod_zone_page_state(zone, NR_ALLOC_BATCH, > high_wmark_pages(zone) - low_wmark_pages(zone) - > @@ -6472,6 +6482,21 @@ int min_free_kbytes_sysctl_handler(struct ctl_table *table, int write, > return 0; > } > > +int watermark_scale_factor_sysctl_handler(struct ctl_table *table, int write, > + void __user *buffer, size_t *length, loff_t *ppos) > +{ > + int rc; > + > + rc = proc_dointvec_minmax(table, write, buffer, length, ppos); > + if (rc) > + return rc; > + > + if (write) > + setup_per_zone_wmarks(); > + > + return 0; > +} > + > #ifdef CONFIG_NUMA > int sysctl_min_unmapped_ratio_sysctl_handler(struct ctl_table *table, int write, > void __user *buffer, size_t *length, loff_t *ppos) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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