From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Ahern Subject: Re: --mmap-pages option seemingly has no effect to help with LOST samples Date: Tue, 12 Jun 2012 15:05:16 -0600 Message-ID: <4FD7AF0C.1030300@gmail.com> References: <4FD7ACB9.70205@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from mail-pb0-f46.google.com ([209.85.160.46]:59569 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752540Ab2FLVFU (ORCPT ); Tue, 12 Jun 2012 17:05:20 -0400 Received: by pbbrp8 with SMTP id rp8so1337193pbb.19 for ; Tue, 12 Jun 2012 14:05:20 -0700 (PDT) In-Reply-To: <4FD7ACB9.70205@us.ibm.com> Sender: linux-perf-users-owner@vger.kernel.org List-ID: To: Maynard Johnson Cc: linux-perf-users@vger.kernel.org On 6/12/12 2:55 PM, Maynard Johnson wrote: > Hi, > On my Intel Core 2 Duo with RHEL 6.2 with the watchdog timer disabled, I'm using perf to collect a CPI profile as follows: > > perf record -e cycles 100000 -e instructions -c 50000 ./memcpyt 500000000 Confused by that command line '-e cycles 100000' is not valid. Missing a -c? If so, -c 100000 followed by -c 50000 means the interval is 50000 for both events; the second one overrides the first. > where 'memcpyt' is a test program that simply does a LOT of memcpy's -- takes about 20 seconds of real time to complete. > > This fails roughly half the time with: > > [ perf record: Woken up 11 times to write data ] > [ perf record: Captured and wrote 4.540 MB perf.data (~198348 samples) ] > Processed 0 events and LOST 872662! > > Check IO/CPU overload! > > I've seen some postings on this list in the past about the LOST events and the suggestion to try the --mmap-pages option. I see from the perf source that the default number of pages to use for mmap'ing the kernel's perf_events data is '8'. I tried going up to 64 pages with little noticeable effect. Additionally, sometimes when I get the LOST samples message, I'll also see the following junk pop up in all of my terminal sessions: > > Message from syslogd@oc3431575272 at Jun 12 15:21:52 ... > kernel:Uhhuh. NMI received for unknown reason 00 on CPU 1. > > Message from syslogd@oc3431575272 at Jun 12 15:21:52 ... > kernel:Do you have a strange power saving mode enabled? > > Message from syslogd@oc3431575272 at Jun 12 15:21:52 ... > kernel:Dazed and confused, but trying to continue I think you are killing your box with NMIs based on the low period (-c arg). I suggest increasing the period. David > > (Not sure, but these syslogd messages may have occurred only when I was running as root.) > > I tried decreasing my sampling rate for both events by half (200000 for cycles and 100000 for instructions), but still got LOST samples, with or without the "--mmap-pages=64" option. Decreasing sampling rate by half again finally did get rid of the LOST samples. > > Questions: > 1) Why doesn't the number of mmap pages seem to have the expected beneficial effect? > 2) Why doesn't the kernel's throttle capabilities prevent the LOST events in the first place? > 3) What's up with the weird syslogd messages? heh. > > I realize none of these may be perf userspace issues, but may be perf_events kernel issues instead. But I thought I'd start out here on this list instead of wading neck-deep into LKML land. > > Thanks. > -Maynard > > -- > To unsubscribe from this list: send the line "unsubscribe linux-perf-users" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html