Re: [PATCH 2/6] mm/page_alloc: Disassociate the pcp->high from pcp->batch
From: Dave Hansen <hidden>
Date: 2021-05-21 21:52:43
Also in:
lkml
On 5/21/21 3:28 AM, Mel Gorman wrote:
Note that in this patch the pcp->high values are adjusted after memory hotplug events, min_free_kbytes adjustments and watermark scale factor adjustments but not CPU hotplug events.
Not that it was a long wait to figure it out, but I'd probably say: "CPU hotplug events are handled later in the series". instead of just saying they're not handled.
Before grep -E "high:|batch" /proc/zoneinfo | tail -2
high: 378
batch: 63
After grep -E "high:|batch" /proc/zoneinfo | tail -2
high: 649
batch: 63You noted the relationship between pcp->high and zone lock contention. Larger ->high values mean less contention. It's probably also worth noting the trend of having more logical CPUs per NUMA node. I have the feeling when this was put in place it wasn't uncommon to have somewhere between 1 and 8 CPUs in a node pounding on a zone. Today, having ~60 is common. I've occasionally resorted to recommending that folks enable hardware features like Sub-NUMA-Clustering [1] since it increases the number of zones and decreases the number of CPUs pounding on each zone lock. 1. https://software.intel.com/content/www/us/en/develop/articles/intel-xeon-processor-scalable-family-technical-overview.html
quoted hunk ↗ jump to hunk
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index a48f305f0381..bf5cdc466e6c 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c@@ -2163,14 +2163,6 @@ void __init page_alloc_init_late(void) /* Block until all are initialised */ wait_for_completion(&pgdat_init_all_done_comp); - /* - * The number of managed pages has changed due to the initialisation - * so the pcpu batch and high limits needs to be updated or the limits - * will be artificially small. - */ - for_each_populated_zone(zone) - zone_pcp_update(zone); - /* * We initialized the rest of the deferred pages. Permanently disable * on-demand struct page initialization.@@ -6594,13 +6586,12 @@ static int zone_batchsize(struct zone *zone) int batch; /* - * The per-cpu-pages pools are set to around 1000th of the - * size of the zone. + * The number of pages to batch allocate is either 0.1%
Probably worth making that "~0.1%" just in case someone goes looking for the /1000 and can't find it.
quoted hunk ↗ jump to hunk
+ * of the zone or 1MB, whichever is smaller. The batch + * size is striking a balance between allocation latency + * and zone lock contention. */ - batch = zone_managed_pages(zone) / 1024; - /* But no more than a meg. */ - if (batch * PAGE_SIZE > 1024 * 1024) - batch = (1024 * 1024) / PAGE_SIZE; + batch = min(zone_managed_pages(zone) >> 10, (1024 * 1024) / PAGE_SIZE); batch /= 4; /* We effectively *= 4 below */ if (batch < 1) batch = 1;@@ -6637,6 +6628,27 @@ static int zone_batchsize(struct zone *zone) #endif } +static int zone_highsize(struct zone *zone) +{ +#ifdef CONFIG_MMU + int high; + int nr_local_cpus; + + /* + * The high value of the pcp is based on the zone low watermark + * when reclaim is potentially active spread across the online + * CPUs local to a zone. Note that early in boot that CPUs may + * not be online yet. + */
FWIW, I like the way the changelog talked about this a bit better, with the goal of avoiding background reclaim even in the face of a bunch of full pcp's.
+ nr_local_cpus = max(1U, cpumask_weight(cpumask_of_node(zone_to_nid(zone)))); + high = low_wmark_pages(zone) / nr_local_cpus;
I'm a little concerned that this might get out of hand on really big nodes with no CPUs. For persistent memory (which we *do* toss into the page allocator for volatile use), we can have multi-terabyte zones with no CPUs in the node. Also, while the CPUs which are on the node are the ones *most* likely to be hitting the ->high limit, we do *keep* a pcp for each possible CPU. So, the amount of memory which can actually be sequestered is num_online_cpus()*high. Right? *That* might really get out of hand if we have nr_local_cpus=1. We might want some overall cap on 'high', or even to scale it differently for the zone-local cpus' pcps versus remote.