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338 lines
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338 lines
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ReStructuredText
=========================================================
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Notes on Analysing Behaviour Using Events and Tracepoints
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=========================================================
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:Author: Mel Gorman (PCL information heavily based on email from Ingo Molnar)
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1. Introduction
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===============
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Tracepoints (see Documentation/trace/tracepoints.rst) can be used without
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creating custom kernel modules to register probe functions using the event
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tracing infrastructure.
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Simplistically, tracepoints represent important events that can be
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taken in conjunction with other tracepoints to build a "Big Picture" of
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what is going on within the system. There are a large number of methods for
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gathering and interpreting these events. Lacking any current Best Practises,
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this document describes some of the methods that can be used.
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This document assumes that debugfs is mounted on /sys/kernel/debug and that
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the appropriate tracing options have been configured into the kernel. It is
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assumed that the PCL tool tools/perf has been installed and is in your path.
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2. Listing Available Events
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===========================
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2.1 Standard Utilities
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----------------------
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All possible events are visible from /sys/kernel/debug/tracing/events. Simply
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calling::
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$ find /sys/kernel/debug/tracing/events -type d
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will give a fair indication of the number of events available.
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2.2 PCL (Performance Counters for Linux)
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----------------------------------------
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Discovery and enumeration of all counters and events, including tracepoints,
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are available with the perf tool. Getting a list of available events is a
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simple case of::
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$ perf list 2>&1 | grep Tracepoint
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ext4:ext4_free_inode [Tracepoint event]
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ext4:ext4_request_inode [Tracepoint event]
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ext4:ext4_allocate_inode [Tracepoint event]
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ext4:ext4_write_begin [Tracepoint event]
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ext4:ext4_ordered_write_end [Tracepoint event]
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[ .... remaining output snipped .... ]
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3. Enabling Events
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==================
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3.1 System-Wide Event Enabling
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------------------------------
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See Documentation/trace/events.rst for a proper description on how events
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can be enabled system-wide. A short example of enabling all events related
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to page allocation would look something like::
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$ for i in `find /sys/kernel/debug/tracing/events -name "enable" | grep mm_`; do echo 1 > $i; done
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3.2 System-Wide Event Enabling with SystemTap
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---------------------------------------------
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In SystemTap, tracepoints are accessible using the kernel.trace() function
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call. The following is an example that reports every 5 seconds what processes
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were allocating the pages.
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::
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global page_allocs
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probe kernel.trace("mm_page_alloc") {
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page_allocs[execname()]++
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}
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function print_count() {
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printf ("%-25s %-s\n", "#Pages Allocated", "Process Name")
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foreach (proc in page_allocs-)
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printf("%-25d %s\n", page_allocs[proc], proc)
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printf ("\n")
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delete page_allocs
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}
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probe timer.s(5) {
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print_count()
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}
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3.3 System-Wide Event Enabling with PCL
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---------------------------------------
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By specifying the -a switch and analysing sleep, the system-wide events
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for a duration of time can be examined.
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::
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$ perf stat -a \
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-e kmem:mm_page_alloc -e kmem:mm_page_free \
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-e kmem:mm_page_free_batched \
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sleep 10
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Performance counter stats for 'sleep 10':
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9630 kmem:mm_page_alloc
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2143 kmem:mm_page_free
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7424 kmem:mm_page_free_batched
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10.002577764 seconds time elapsed
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Similarly, one could execute a shell and exit it as desired to get a report
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at that point.
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3.4 Local Event Enabling
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------------------------
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Documentation/trace/ftrace.rst describes how to enable events on a per-thread
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basis using set_ftrace_pid.
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3.5 Local Event Enablement with PCL
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-----------------------------------
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Events can be activated and tracked for the duration of a process on a local
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basis using PCL such as follows.
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::
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$ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free \
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-e kmem:mm_page_free_batched ./hackbench 10
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Time: 0.909
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Performance counter stats for './hackbench 10':
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17803 kmem:mm_page_alloc
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12398 kmem:mm_page_free
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4827 kmem:mm_page_free_batched
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0.973913387 seconds time elapsed
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4. Event Filtering
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==================
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Documentation/trace/ftrace.rst covers in-depth how to filter events in
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ftrace. Obviously using grep and awk of trace_pipe is an option as well
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as any script reading trace_pipe.
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5. Analysing Event Variances with PCL
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=====================================
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Any workload can exhibit variances between runs and it can be important
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to know what the standard deviation is. By and large, this is left to the
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performance analyst to do it by hand. In the event that the discrete event
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occurrences are useful to the performance analyst, then perf can be used.
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::
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$ perf stat --repeat 5 -e kmem:mm_page_alloc -e kmem:mm_page_free
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-e kmem:mm_page_free_batched ./hackbench 10
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Time: 0.890
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Time: 0.895
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Time: 0.915
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Time: 1.001
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Time: 0.899
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Performance counter stats for './hackbench 10' (5 runs):
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16630 kmem:mm_page_alloc ( +- 3.542% )
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11486 kmem:mm_page_free ( +- 4.771% )
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4730 kmem:mm_page_free_batched ( +- 2.325% )
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0.982653002 seconds time elapsed ( +- 1.448% )
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In the event that some higher-level event is required that depends on some
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aggregation of discrete events, then a script would need to be developed.
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Using --repeat, it is also possible to view how events are fluctuating over
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time on a system-wide basis using -a and sleep.
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::
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$ perf stat -e kmem:mm_page_alloc -e kmem:mm_page_free \
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-e kmem:mm_page_free_batched \
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-a --repeat 10 \
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sleep 1
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Performance counter stats for 'sleep 1' (10 runs):
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1066 kmem:mm_page_alloc ( +- 26.148% )
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182 kmem:mm_page_free ( +- 5.464% )
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890 kmem:mm_page_free_batched ( +- 30.079% )
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1.002251757 seconds time elapsed ( +- 0.005% )
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6. Higher-Level Analysis with Helper Scripts
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============================================
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When events are enabled the events that are triggering can be read from
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/sys/kernel/debug/tracing/trace_pipe in human-readable format although binary
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options exist as well. By post-processing the output, further information can
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be gathered on-line as appropriate. Examples of post-processing might include
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- Reading information from /proc for the PID that triggered the event
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- Deriving a higher-level event from a series of lower-level events.
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- Calculating latencies between two events
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Documentation/trace/postprocess/trace-pagealloc-postprocess.pl is an example
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script that can read trace_pipe from STDIN or a copy of a trace. When used
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on-line, it can be interrupted once to generate a report without exiting
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and twice to exit.
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Simplistically, the script just reads STDIN and counts up events but it
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also can do more such as
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- Derive high-level events from many low-level events. If a number of pages
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are freed to the main allocator from the per-CPU lists, it recognises
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that as one per-CPU drain even though there is no specific tracepoint
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for that event
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- It can aggregate based on PID or individual process number
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- In the event memory is getting externally fragmented, it reports
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on whether the fragmentation event was severe or moderate.
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- When receiving an event about a PID, it can record who the parent was so
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that if large numbers of events are coming from very short-lived
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processes, the parent process responsible for creating all the helpers
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can be identified
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7. Lower-Level Analysis with PCL
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================================
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There may also be a requirement to identify what functions within a program
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were generating events within the kernel. To begin this sort of analysis, the
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data must be recorded. At the time of writing, this required root:
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::
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$ perf record -c 1 \
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-e kmem:mm_page_alloc -e kmem:mm_page_free \
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-e kmem:mm_page_free_batched \
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./hackbench 10
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Time: 0.894
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[ perf record: Captured and wrote 0.733 MB perf.data (~32010 samples) ]
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Note the use of '-c 1' to set the event period to sample. The default sample
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period is quite high to minimise overhead but the information collected can be
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very coarse as a result.
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This record outputted a file called perf.data which can be analysed using
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perf report.
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::
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$ perf report
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# Samples: 30922
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#
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# Overhead Command Shared Object
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# ........ ......... ................................
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#
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87.27% hackbench [vdso]
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6.85% hackbench /lib/i686/cmov/libc-2.9.so
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2.62% hackbench /lib/ld-2.9.so
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1.52% perf [vdso]
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1.22% hackbench ./hackbench
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0.48% hackbench [kernel]
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0.02% perf /lib/i686/cmov/libc-2.9.so
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0.01% perf /usr/bin/perf
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0.01% perf /lib/ld-2.9.so
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0.00% hackbench /lib/i686/cmov/libpthread-2.9.so
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#
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# (For more details, try: perf report --sort comm,dso,symbol)
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#
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According to this, the vast majority of events triggered on events
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within the VDSO. With simple binaries, this will often be the case so let's
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take a slightly different example. In the course of writing this, it was
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noticed that X was generating an insane amount of page allocations so let's look
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at it:
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::
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$ perf record -c 1 -f \
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-e kmem:mm_page_alloc -e kmem:mm_page_free \
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-e kmem:mm_page_free_batched \
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-p `pidof X`
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This was interrupted after a few seconds and
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::
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$ perf report
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# Samples: 27666
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#
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# Overhead Command Shared Object
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# ........ ....... .......................................
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#
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51.95% Xorg [vdso]
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47.95% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1
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0.09% Xorg /lib/i686/cmov/libc-2.9.so
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0.01% Xorg [kernel]
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#
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# (For more details, try: perf report --sort comm,dso,symbol)
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#
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So, almost half of the events are occurring in a library. To get an idea which
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symbol:
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::
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$ perf report --sort comm,dso,symbol
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# Samples: 27666
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#
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# Overhead Command Shared Object Symbol
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# ........ ....... ....................................... ......
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#
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51.95% Xorg [vdso] [.] 0x000000ffffe424
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47.93% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] pixmanFillsse2
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0.09% Xorg /lib/i686/cmov/libc-2.9.so [.] _int_malloc
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0.01% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] pixman_region32_copy_f
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0.01% Xorg [kernel] [k] read_hpet
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0.01% Xorg /opt/gfx-test/lib/libpixman-1.so.0.13.1 [.] get_fast_path
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0.00% Xorg [kernel] [k] ftrace_trace_userstack
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To see where within the function pixmanFillsse2 things are going wrong:
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::
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$ perf annotate pixmanFillsse2
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[ ... ]
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0.00 : 34eeb: 0f 18 08 prefetcht0 (%eax)
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: }
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:
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: extern __inline void __attribute__((__gnu_inline__, __always_inline__, _
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: _mm_store_si128 (__m128i *__P, __m128i __B) : {
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: *__P = __B;
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12.40 : 34eee: 66 0f 7f 80 40 ff ff movdqa %xmm0,-0xc0(%eax)
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0.00 : 34ef5: ff
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12.40 : 34ef6: 66 0f 7f 80 50 ff ff movdqa %xmm0,-0xb0(%eax)
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0.00 : 34efd: ff
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12.39 : 34efe: 66 0f 7f 80 60 ff ff movdqa %xmm0,-0xa0(%eax)
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0.00 : 34f05: ff
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12.67 : 34f06: 66 0f 7f 80 70 ff ff movdqa %xmm0,-0x90(%eax)
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0.00 : 34f0d: ff
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12.58 : 34f0e: 66 0f 7f 40 80 movdqa %xmm0,-0x80(%eax)
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12.31 : 34f13: 66 0f 7f 40 90 movdqa %xmm0,-0x70(%eax)
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12.40 : 34f18: 66 0f 7f 40 a0 movdqa %xmm0,-0x60(%eax)
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12.31 : 34f1d: 66 0f 7f 40 b0 movdqa %xmm0,-0x50(%eax)
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At a glance, it looks like the time is being spent copying pixmaps to
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the card. Further investigation would be needed to determine why pixmaps
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are being copied around so much but a starting point would be to take an
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ancient build of libpixmap out of the library path where it was totally
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forgotten about from months ago!
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