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Message-Id: <1249666815-28784-7-git-send-email-mel@csn.ul.ie> Date: Fri, 7 Aug 2009 18:40:15 +0100 From: Mel Gorman <mel@....ul.ie> To: Larry Woodman <lwoodman@...hat.com>, Ingo Molnar <mingo@...e.hu>, Andrew Morton <akpm@...ux-foundation.org> Cc: riel@...hat.com, Peter Zijlstra <peterz@...radead.org>, LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org, Mel Gorman <mel@....ul.ie> Subject: [PATCH 6/6] tracing, documentation: Add a document on the kmem tracepoints Knowing tracepoints exist is not quite the same as knowing what they should be used for. This patch adds a document giving a basic description of the kmem tracepoints and why they might be useful to a performance analyst. Signed-off-by: Mel Gorman <mel@....ul.ie> --- Documentation/trace/events-kmem.txt | 107 +++++++++++++++++++++++++++++++++++ 1 files changed, 107 insertions(+), 0 deletions(-) create mode 100644 Documentation/trace/events-kmem.txt diff --git a/Documentation/trace/events-kmem.txt b/Documentation/trace/events-kmem.txt new file mode 100644 index 0000000..6ef2a86 --- /dev/null +++ b/Documentation/trace/events-kmem.txt @@ -0,0 +1,107 @@ + Subsystem Trace Points: kmem + +The tracing system kmem captures events related to object and page allocation +within the kernel. Broadly speaking there are four major subheadings. + + o Slab allocation of small objects of unknown type (kmalloc) + o Slab allocation of small objects of known type + o Page allocation + o Per-CPU Allocator Activity + o External Fragmentation + +This document will describe what each of the tracepoints are and why they +might be useful. + +1. Slab allocation of small objects of unknown type +=================================================== +kmalloc call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s +kmalloc_node call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s node=%d +kfree call_site=%lx ptr=%p + +Heavy activity for these events may indicate that a specific cache is +justified, particularly if kmalloc slab pages are getting significantly +internal fragmented as a result of the allocation pattern. By correlating +kmalloc with kfree, it may be possible to identify memory leaks and where +the allocation sites were. + + +2. Slab allocation of small objects of known type +================================================= +kmem_cache_alloc call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s +kmem_cache_alloc_node call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s node=%d +kmem_cache_free call_site=%lx ptr=%p + +These events are similar in usage to the kmalloc-related events except that +it is likely easier to pin the event down to a specific cache. At the time +of writing, no information is available on what slab is being allocated from, +but the call_site can usually be used to extrapolate that information + +3. Page allocation +================== +mm_page_alloc page=%p pfn=%lu order=%d migratetype=%d gfp_flags=%s +mm_page_alloc_zone_locked page=%p pfn=%lu order=%u migratetype=%d cpu=%d percpu_refill=%d +mm_page_free_direct page=%p pfn=%lu order=%d +mm_pagevec_free page=%p pfn=%lu order=%d cold=%d + +These four events deal with page allocation and freeing. mm_page_alloc is +a simple indicator of page allocator activity. Pages may be allocated from +the per-CPU allocator (high performance) or the buddy allocator. + +If pages are allocated directly from the buddy allocator, the +mm_page_alloc_zone_locked event is triggered. This event is important as high +amounts of activity imply high activity on the zone->lock. Taking this lock +impairs performance by disabling interrupts, dirtying cache lines between +CPUs and serialising many CPUs. + +When a page is freed directly by the caller, the mm_page_free_direct event +is triggered. Significant amounts of activity here could indicate that the +callers should be batching their activities. + +When pages are freed using a pagevec, the mm_pagevec_free is +triggered. Broadly speaking, pages are taken off the LRU lock in bulk and +freed in batch with a pagevec. Significant amounts of activity here could +indicate that the system is under memory pressure and can also indicate +contention on the zone->lru_lock. + +4. Per-CPU Allocator Activity +============================= +mm_page_alloc_zone_locked page=%p pfn=%lu order=%u migratetype=%d cpu=%d percpu_refill=%d +mm_page_pcpu_drain page=%p pfn=%lu order=%d cpu=%d migratetype=%d + +In front of the page allocator is a per-cpu page allocator. It exists only +for order-0 pages, reduces contention on the zone->lock and reduces the +amount of writing on struct page. + +When a per-CPU list is empty or pages of the wrong type are allocated, +the zone->lock will be taken once and the per-CPU list refilled. The event +triggered is mm_page_alloc_zone_locked for each page allocated with the +event indicating whether it is for a percpu_refill or not. + +When the per-CPU list is too full, a number of pages are freed, each one +which triggers a mm_page_pcpu_drain event. + +The individual nature of the events are so that pages can be tracked +between allocation and freeing. A number of drain or refill pages that occur +consecutively imply the zone->lock being taken once. Large amounts of PCP +refills and drains could imply an imbalance between CPUs where too much work +is being concentrated in one place. It could also indicate that the per-CPU +lists should be a larger size. Finally, large amounts of refills on one CPU +and drains on another could be a factor in causing large amounts of cache +line bounces due to writes between CPUs and worth investigating if pages +can be allocated and freed on the same CPU through some algorithm change. + +5. External Fragmentation +========================= +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 + +External fragmentation affects whether a high-order allocation will be +successful or not. For some types of hardware, this is important although +it is avoided where possible. If the system is using huge pages and needs +to be able to resize the pool over the lifetime of the system, this value +is important. + +Large numbers of this event implies that memory is fragmenting and +high-order allocations will start failing at some time in the future. One +means of reducing the occurange of this event is to increase the size of +min_free_kbytes in increments of 3*pageblock_size*nr_online_nodes where +pageblock_size is usually the size of the default hugepage size. -- 1.6.3.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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