About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / trace / events-kmem.txt




Custom Search

Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1				Subsystem Trace Points: kmem
2	
3	The kmem tracing system captures events related to object and page allocation
4	within the kernel. Broadly speaking there are five major subheadings.
5	
6	  o Slab allocation of small objects of unknown type (kmalloc)
7	  o Slab allocation of small objects of known type
8	  o Page allocation
9	  o Per-CPU Allocator Activity
10	  o External Fragmentation
11	
12	This document describes what each of the tracepoints is and why they
13	might be useful.
14	
15	1. Slab allocation of small objects of unknown type
16	===================================================
17	kmalloc		call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s
18	kmalloc_node	call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s node=%d
19	kfree		call_site=%lx ptr=%p
20	
21	Heavy activity for these events may indicate that a specific cache is
22	justified, particularly if kmalloc slab pages are getting significantly
23	internal fragmented as a result of the allocation pattern. By correlating
24	kmalloc with kfree, it may be possible to identify memory leaks and where
25	the allocation sites were.
26	
27	
28	2. Slab allocation of small objects of known type
29	=================================================
30	kmem_cache_alloc	call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s
31	kmem_cache_alloc_node	call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s node=%d
32	kmem_cache_free		call_site=%lx ptr=%p
33	
34	These events are similar in usage to the kmalloc-related events except that
35	it is likely easier to pin the event down to a specific cache. At the time
36	of writing, no information is available on what slab is being allocated from,
37	but the call_site can usually be used to extrapolate that information.
38	
39	3. Page allocation
40	==================
41	mm_page_alloc		  page=%p pfn=%lu order=%d migratetype=%d gfp_flags=%s
42	mm_page_alloc_zone_locked page=%p pfn=%lu order=%u migratetype=%d cpu=%d percpu_refill=%d
43	mm_page_free		  page=%p pfn=%lu order=%d
44	mm_page_free_batched	  page=%p pfn=%lu order=%d cold=%d
45	
46	These four events deal with page allocation and freeing. mm_page_alloc is
47	a simple indicator of page allocator activity. Pages may be allocated from
48	the per-CPU allocator (high performance) or the buddy allocator.
49	
50	If pages are allocated directly from the buddy allocator, the
51	mm_page_alloc_zone_locked event is triggered. This event is important as high
52	amounts of activity imply high activity on the zone->lock. Taking this lock
53	impairs performance by disabling interrupts, dirtying cache lines between
54	CPUs and serialising many CPUs.
55	
56	When a page is freed directly by the caller, the only mm_page_free event
57	is triggered. Significant amounts of activity here could indicate that the
58	callers should be batching their activities.
59	
60	When pages are freed in batch, the also mm_page_free_batched is triggered.
61	Broadly speaking, pages are taken off the LRU lock in bulk and
62	freed in batch with a page list. Significant amounts of activity here could
63	indicate that the system is under memory pressure and can also indicate
64	contention on the zone->lru_lock.
65	
66	4. Per-CPU Allocator Activity
67	=============================
68	mm_page_alloc_zone_locked	page=%p pfn=%lu order=%u migratetype=%d cpu=%d percpu_refill=%d
69	mm_page_pcpu_drain		page=%p pfn=%lu order=%d cpu=%d migratetype=%d
70	
71	In front of the page allocator is a per-cpu page allocator. It exists only
72	for order-0 pages, reduces contention on the zone->lock and reduces the
73	amount of writing on struct page.
74	
75	When a per-CPU list is empty or pages of the wrong type are allocated,
76	the zone->lock will be taken once and the per-CPU list refilled. The event
77	triggered is mm_page_alloc_zone_locked for each page allocated with the
78	event indicating whether it is for a percpu_refill or not.
79	
80	When the per-CPU list is too full, a number of pages are freed, each one
81	which triggers a mm_page_pcpu_drain event.
82	
83	The individual nature of the events is so that pages can be tracked
84	between allocation and freeing. A number of drain or refill pages that occur
85	consecutively imply the zone->lock being taken once. Large amounts of per-CPU
86	refills and drains could imply an imbalance between CPUs where too much work
87	is being concentrated in one place. It could also indicate that the per-CPU
88	lists should be a larger size. Finally, large amounts of refills on one CPU
89	and drains on another could be a factor in causing large amounts of cache
90	line bounces due to writes between CPUs and worth investigating if pages
91	can be allocated and freed on the same CPU through some algorithm change.
92	
93	5. External Fragmentation
94	=========================
95	mm_page_alloc_extfrag		page=%p pfn=%lu alloc_order=%d fallback_order=%d pageblock_order=%d alloc_migratetype=%d fallback_migratetype=%d fragmenting=%d change_ownership=%d
96	
97	External fragmentation affects whether a high-order allocation will be
98	successful or not. For some types of hardware, this is important although
99	it is avoided where possible. If the system is using huge pages and needs
100	to be able to resize the pool over the lifetime of the system, this value
101	is important.
102	
103	Large numbers of this event implies that memory is fragmenting and
104	high-order allocations will start failing at some time in the future. One
105	means of reducing the occurrence of this event is to increase the size of
106	min_free_kbytes in increments of 3*pageblock_size*nr_online_nodes where
107	pageblock_size is usually the size of the default hugepage size.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.