Re: perf complains losting events

[David Ahern <dsahern@gmail.com>]

On 08/03/2011 04:54 AM, Peter Zijlstra wrote:
> 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.

Increasing the number of memory pages helps too (-m arg).

David

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