Messages in this thread | | | Subject | Re: perf complains losting events | From | Peter Zijlstra <> | Date | Wed, 03 Aug 2011 12:54:11 +0200 |
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On Wed, 2011-08-03 at 18:28 +0800, Han Pingtian wrote: > Hi, > > I find there is a comment about losting events: > > /* > * The kernel collects the number of events it couldn't send in a stretch and > * when possible sends this number in a PERF_RECORD_LOST event. The number of > * such "chunks" of lost events is stored in .nr_events[PERF_EVENT_LOST] while > * total_lost tells exactly how many events the kernel in fact lost, i.e. it is > * the sum of all struct lost_event.lost fields reported. > * > * The total_period is needed because by default auto-freq is used, so > * multipling nr_events[PERF_EVENT_SAMPLE] by a frequency isn't possible to get > * the total number of low level events, it is necessary to to sum all struct > * sample_event.period and stash the result in total_period. > */ > > So my question is, whether the losting of events is a problem? > I have saw it many times: > > [root@hp-dl580g7-01 perf]# ./perf kmem record sleep 1 > [ perf record: Woken up 0 times to write data ] > [ perf record: Captured and wrote 21.789 MB perf.data (~951977 samples) > ] > Processed 0 events and LOST 76148! > > Check IO/CPU overload! > > [root@hp-dl580g7-01 perf]# ./perf kmem stat > Processed 0 events and LOST 76148! > > Check IO/CPU overload! > > > SUMMARY > ======= > Total bytes requested: 5725028 > Total bytes allocated: 6291512 > Total bytes wasted on internal fragmentation: 566484 > Internal fragmentation: 9.003941% > Cross CPU allocations: 28/84295
Just means there's too many event to process, if you run record as a realtime task its less:
$ perf record -a -r 1 -R -f -c 1 -e kmem:kmalloc -e kmem:kmalloc_node -e kmem:kfree -e kmem:kmem_cache_alloc -e kmem:kmem_cache_alloc_node -e kmem:kmem_cache_free -- sleep 2 [ perf record: Woken up 2 times to write data ] [ perf record: Captured and wrote 3.642 MB perf.data (~159113 samples) ] Processed 0 events and LOST 7213!
On the question on if its a problem, that very much depends on what you want to do and what kind of precision you need from you data..
I suspect that once we start writing one file per cpu this again will improve somewhat. Acme was going to work on that.. dunno what his plans are.
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