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Clarke" To: Ian Rogers Cc: Andi Kleen , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Alexei Starovoitov , Daniel Borkmann , Martin KaFai Lau , Song Liu , Yonghong Song , Andrii Nakryiko , John Fastabend , KP Singh , Kajol Jain , John Garry , Jin Yao , Kan Liang , Cong Wang , Kim Phillips , LKML , Networking , bpf , linux-perf-users , Stephane Eranian Subject: Re: [RFC PATCH 0/7] Share events between metrics Message-ID: <20201215150812.GA38786@li-24c3614c-2adc-11b2-a85c-85f334518bdb.ibm.com> References: <20200507081436.49071-1-irogers@google.com> <20200507174835.GB3538@tassilo.jf.intel.com> <20200507214652.GC3538@tassilo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.343,18.0.737 definitions=2020-12-15_12:2020-12-15,2020-12-15 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxscore=0 clxscore=1011 suspectscore=0 impostorscore=0 lowpriorityscore=0 priorityscore=1501 mlxlogscore=999 bulkscore=0 adultscore=0 malwarescore=0 spamscore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2012150104 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 07, 2020 at 10:43:43PM -0700, Ian Rogers wrote: > On Thu, May 7, 2020 at 2:47 PM Andi Kleen wrote: > > > > > > - without this change events within a metric may get scheduled > > > > together, after they may appear as part of a larger group and be > > > > multiplexed at different times, lowering accuracy - however, less > > > > multiplexing may compensate for this. Does mutiplexing somewhat related events at different times actually reduce accuracy, or is it just more likely to give that appearance? It seems that perf measurements are only useful if the workload is in a fairly steady state. If there is some wobbling, then measuring at the same time is more accurate for the periods where the events are being measured simultaneously, but may be far off for when they are not being measured at all. Spreading them out over a longer duration may actually increase accuracy by sampling over more varied intervals. Or, is the concern more about trying to time-slice the results in a fairly granular way and expecting accurate results then? (Or, maybe my ignorance is showing again. :-) PC