From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ej1-f73.google.com (mail-ej1-f73.google.com [209.85.218.73]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BD5CA211A2E for ; Thu, 13 Feb 2025 09:08:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.218.73 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739437732; cv=none; b=XBs1bMqQP8cGOPpvh4lZe2sw+VFh3ZDDCoQCVS457BQe5RKZdB/sN24toqz5aBdTBiHI42buxCS4vIaMfZSgo6cLnWmuZcnRqCI8gM8KGAk2xXAJPoOodIbS+uAsZay/sxIINT3i+nRGhGFdz/9+88ShACSQwHQV5MxKp4TvORU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739437732; c=relaxed/simple; bh=hHsYh4Wj2aKIk+sbbDk+eoksuvTL9kE1wTEO+n06vms=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=MGPwKaJWK50ZgsuETc0TD/vkZIBtFrf5M3UvP1BdWtILfZ1e+tHqZbugpeG8A7PaFyRD+gWc5117KYo2e6r6BjHzQSmXqKmF+LG3pbeXHMfA8EuQOqZKM/97fJZMT7sjLzujHZYzG6nzsffRUgRvuDPi/n67a5pQ798XuGvdcis= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--dvyukov.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=BhXcywJ8; arc=none smtp.client-ip=209.85.218.73 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--dvyukov.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="BhXcywJ8" Received: by mail-ej1-f73.google.com with SMTP id a640c23a62f3a-ab7d2d60e88so59853566b.1 for ; Thu, 13 Feb 2025 01:08:49 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1739437728; x=1740042528; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=T2iUxOYgcCaQTSWGHwDxztwJgFH0wzbbb/8+hafzV1k=; b=BhXcywJ8sUsLPqM+lEwgE4a9JdDuZo7iRB4JA9R1yUXlJKxHjrTU6jNyAm0kDQA0Ez vMF1GNbXDmNGvDQRuCBkYbqai6X4qqyGyJ7Ksh/lgQHeyohIIC5XUFDPgvYK7u4zv7Dt IWCoA7uy4bg1eVQmChDo0r82QKaDj1wsNf5/A6wNjZ+3fGLfccAKdFzFhHQuop5Xmn9s 1vI3PtQiyczagguw4KvkGQrgKtqWl5jOniRFCh/CS9zkRw076+Xh4Ye5CQ8ZNPHkMf3A 1ldRnxSy63HnlfiQe5/+jvCETK6lCHnBiMse50nQKFprHPKVjEKtZ6D8V/1vnmJaSY4L 61Pw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1739437728; x=1740042528; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=T2iUxOYgcCaQTSWGHwDxztwJgFH0wzbbb/8+hafzV1k=; b=I1Zns2WC02AJNTG0DLnYTu3wdQy8U5qrEO3YbFRRNyLMluiBrcfkljT8GA9sQ9l3GI XuwSusNY4fea1FuHN3fZLggnetEAfiZbd75x0O/mWhPmDj2xOMolmQ1wyvHEwGFseNNY 10pig4bVDFgJRVUDF74x/ddIaZ2+D7ZNLYWh450sWNAV0LsucmjKAQRonWu1Y/lhjpgf 8g+x6mAGdJC2FxOpj4Cbhfz0Gvh+IjiWyYVS1k40uWOnznLuKda2wshWUOCNFMO0gBGm tHpI7UqbScyPPAoHbYMW+E5sTG0KUPqgkmFoyiBR7klI1KL+kSRcn1fz6EGcaWtfOa8z jd6w== X-Gm-Message-State: AOJu0Yzgm68L1vMqW5Z+Gh/vwIkpQbcFmZgSDAVmPxxBco6Geiaz40is WLiOiB89w+/obLODIpKMaMSyry2K+i2qFZ3KHzon1P0FlMuh4scQ9JVqyMjOMt4bdX9EKIC4na/ BFBXxQw== X-Google-Smtp-Source: AGHT+IFpfvIQ0Sg4w0j91B9TGflRTIxP/RoKsnyIB+IfuX0sf2113hN1CEdmW8p2me+OaamqbmaPRnW3Kgnv X-Received: from ejbb16.prod.google.com ([2002:a17:906:30d0:b0:ab7:b8ec:cf22]) (user=dvyukov job=prod-delivery.src-stubby-dispatcher) by 2002:a17:907:2da1:b0:ab6:f06b:4a26 with SMTP id a640c23a62f3a-ab7f33e0e2amr761382466b.34.1739437728204; Thu, 13 Feb 2025 01:08:48 -0800 (PST) Date: Thu, 13 Feb 2025 10:08:20 +0100 In-Reply-To: Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: X-Mailer: git-send-email 2.48.1.502.g6dc24dfdaf-goog Message-ID: Subject: [PATCH v7 7/9] perf report: Add latency and parallelism profiling documentation From: Dmitry Vyukov To: namhyung@kernel.org, irogers@google.com, acme@kernel.org, ak@linux.intel.com Cc: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, Dmitry Vyukov Content-Type: text/plain; charset="UTF-8" Describe latency and parallelism profiling, related flags, and differences with the currently only supported CPU-consumption-centric profiling. Signed-off-by: Dmitry Vyukov Cc: Namhyung Kim Cc: Arnaldo Carvalho de Melo Cc: Ian Rogers Cc: linux-perf-users@vger.kernel.org Cc: linux-kernel@vger.kernel.org --- Changes in v7: - add tips.txt example with time/time-quantum --- .../callchain-overhead-calculation.txt | 5 +- .../cpu-and-latency-overheads.txt | 85 +++++++++++++++++++ tools/perf/Documentation/perf-report.txt | 49 +++++++---- tools/perf/Documentation/tips.txt | 4 + 4 files changed, 124 insertions(+), 19 deletions(-) diff --git a/tools/perf/Documentation/callchain-overhead-calculation.txt b/tools/perf/Documentation/callchain-overhead-calculation.txt index 1a757927195ed..e0202bf5bd1a0 100644 --- a/tools/perf/Documentation/callchain-overhead-calculation.txt +++ b/tools/perf/Documentation/callchain-overhead-calculation.txt @@ -1,7 +1,8 @@ Overhead calculation -------------------- -The overhead can be shown in two columns as 'Children' and 'Self' when -perf collects callchains. The 'self' overhead is simply calculated by +The CPU overhead can be shown in two columns as 'Children' and 'Self' +when perf collects callchains (and corresponding 'Wall' columns for +wall-clock overhead). The 'self' overhead is simply calculated by adding all period values of the entry - usually a function (symbol). This is the value that perf shows traditionally and sum of all the 'self' overhead values should be 100%. diff --git a/tools/perf/Documentation/cpu-and-latency-overheads.txt b/tools/perf/Documentation/cpu-and-latency-overheads.txt new file mode 100644 index 0000000000000..3b6d637054651 --- /dev/null +++ b/tools/perf/Documentation/cpu-and-latency-overheads.txt @@ -0,0 +1,85 @@ +CPU and latency overheads +------------------------- +There are two notions of time: wall-clock time and CPU time. +For a single-threaded program, or a program running on a single-core machine, +these notions are the same. However, for a multi-threaded/multi-process program +running on a multi-core machine, these notions are significantly different. +Each second of wall-clock time we have number-of-cores seconds of CPU time. +Perf can measure overhead for both of these times (shown in 'overhead' and +'latency' columns for CPU and wall-clock time correspondingly). + +Optimizing CPU overhead is useful to improve 'throughput', while optimizing +latency overhead is useful to improve 'latency'. It's important to understand +which one is useful in a concrete situation at hand. For example, the former +may be useful to improve max throughput of a CI build server that runs on 100% +CPU utilization, while the latter may be useful to improve user-perceived +latency of a single interactive program build. +These overheads may be significantly different in some cases. For example, +consider a program that executes function 'foo' for 9 seconds with 1 thread, +and then executes function 'bar' for 1 second with 128 threads (consumes +128 seconds of CPU time). The CPU overhead is: 'foo' - 6.6%, 'bar' - 93.4%. +While the latency overhead is: 'foo' - 90%, 'bar' - 10%. If we try to optimize +running time of the program looking at the (wrong in this case) CPU overhead, +we would concentrate on the function 'bar', but it can yield only 10% running +time improvement at best. + +By default, perf shows only CPU overhead. To show latency overhead, use +'perf record --latency' and 'perf report': + +----------------------------------- +Overhead Latency Command + 93.88% 25.79% cc1 + 1.90% 39.87% gzip + 0.99% 10.16% dpkg-deb + 0.57% 1.00% as + 0.40% 0.46% sh +----------------------------------- + +To sort by latency overhead, use 'perf report --latency': + +----------------------------------- +Latency Overhead Command + 39.87% 1.90% gzip + 25.79% 93.88% cc1 + 10.16% 0.99% dpkg-deb + 4.17% 0.29% git + 2.81% 0.11% objtool +----------------------------------- + +To get insight into the difference between the overheads, you may check +parallelization histogram with '--sort=latency,parallelism,comm,symbol --hierarchy' +flags. It shows fraction of (wall-clock) time the workload utilizes different +numbers of cores ('Parallelism' column). For example, in the following case +the workload utilizes only 1 core most of the time, but also has some +highly-parallel phases, which explains significant difference between +CPU and wall-clock overheads: + +----------------------------------- + Latency Overhead Parallelism / Command / Symbol ++ 56.98% 2.29% 1 ++ 16.94% 1.36% 2 ++ 4.00% 20.13% 125 ++ 3.66% 18.25% 124 ++ 3.48% 17.66% 126 ++ 3.26% 0.39% 3 ++ 2.61% 12.93% 123 +----------------------------------- + +By expanding corresponding lines, you may see what commands/functions run +at the given parallelism level: + +----------------------------------- + Latency Overhead Parallelism / Command / Symbol +- 56.98% 2.29% 1 + 32.80% 1.32% gzip + 4.46% 0.18% cc1 + 2.81% 0.11% objtool + 2.43% 0.10% dpkg-source + 2.22% 0.09% ld + 2.10% 0.08% dpkg-genchanges +----------------------------------- + +To see the normal function-level profile for particular parallelism levels +(number of threads actively running on CPUs), you may use '--parallelism' +filter. For example, to see the profile only for low parallelism phases +of a workload use '--latency --parallelism=1-2' flags. diff --git a/tools/perf/Documentation/perf-report.txt b/tools/perf/Documentation/perf-report.txt index 66794131aec48..3376c47105750 100644 --- a/tools/perf/Documentation/perf-report.txt +++ b/tools/perf/Documentation/perf-report.txt @@ -44,7 +44,7 @@ OPTIONS --comms=:: Only consider symbols in these comms. CSV that understands file://filename entries. This option will affect the percentage of - the overhead column. See --percentage for more info. + the overhead and latency columns. See --percentage for more info. --pid=:: Only show events for given process ID (comma separated list). @@ -54,12 +54,12 @@ OPTIONS --dsos=:: Only consider symbols in these dsos. CSV that understands file://filename entries. This option will affect the percentage of - the overhead column. See --percentage for more info. + the overhead and latency columns. See --percentage for more info. -S:: --symbols=:: Only consider these symbols. CSV that understands file://filename entries. This option will affect the percentage of - the overhead column. See --percentage for more info. + the overhead and latency columns. See --percentage for more info. --symbol-filter=:: Only show symbols that match (partially) with this filter. @@ -68,6 +68,16 @@ OPTIONS --hide-unresolved:: Only display entries resolved to a symbol. +--parallelism:: + Only consider these parallelism levels. Parallelism level is the number + of threads that actively run on CPUs at the time of sample. The flag + accepts single number, comma-separated list, and ranges (for example: + "1", "7,8", "1,64-128"). This is useful in understanding what a program + is doing during sequential/low-parallelism phases as compared to + high-parallelism phases. This option will affect the percentage of + the overhead and latency columns. See --percentage for more info. + Also see the `CPU and latency overheads' section for more details. + --latency:: Show latency-centric profile rather than the default CPU-consumption-centric profile @@ -92,6 +102,7 @@ OPTIONS entries are displayed as "[other]". - cpu: cpu number the task ran at the time of sample - socket: processor socket number the task ran at the time of sample + - parallelism: number of running threads at the time of sample - srcline: filename and line number executed at the time of sample. The DWARF debugging info must be provided. - srcfile: file name of the source file of the samples. Requires dwarf @@ -102,12 +113,14 @@ OPTIONS - cgroup_id: ID derived from cgroup namespace device and inode numbers. - cgroup: cgroup pathname in the cgroupfs. - transaction: Transaction abort flags. - - overhead: Overhead percentage of sample - - overhead_sys: Overhead percentage of sample running in system mode - - overhead_us: Overhead percentage of sample running in user mode - - overhead_guest_sys: Overhead percentage of sample running in system mode + - overhead: CPU overhead percentage of sample. + - latency: latency (wall-clock) overhead percentage of sample. + See the `CPU and latency overheads' section for more details. + - overhead_sys: CPU overhead percentage of sample running in system mode + - overhead_us: CPU overhead percentage of sample running in user mode + - overhead_guest_sys: CPU overhead percentage of sample running in system mode on guest machine - - overhead_guest_us: Overhead percentage of sample running in user mode on + - overhead_guest_us: CPU overhead percentage of sample running in user mode on guest machine - sample: Number of sample - period: Raw number of event count of sample @@ -130,8 +143,8 @@ OPTIONS - weight2: Average value of event specific weight (2nd field of weight_struct). - weight3: Average value of event specific weight (3rd field of weight_struct). - By default, comm, dso and symbol keys are used. - (i.e. --sort comm,dso,symbol) + By default, overhead, comm, dso and symbol keys are used. + (i.e. --sort overhead,comm,dso,symbol). If --branch-stack option is used, following sort keys are also available: @@ -206,9 +219,9 @@ OPTIONS --fields=:: Specify output field - multiple keys can be specified in CSV format. Following fields are available: - overhead, overhead_sys, overhead_us, overhead_children, sample, period, - weight1, weight2, weight3, ins_lat, p_stage_cyc and retire_lat. The - last 3 names are alias for the corresponding weights. When the weight + overhead, latency, overhead_sys, overhead_us, overhead_children, sample, + period, weight1, weight2, weight3, ins_lat, p_stage_cyc and retire_lat. + The last 3 names are alias for the corresponding weights. When the weight fields are used, they will show the average value of the weight. Also it can contain any sort key(s). @@ -294,7 +307,7 @@ OPTIONS Accumulate callchain of children to parent entry so that then can show up in the output. The output will have a new "Children" column and will be sorted on the data. It requires callchains are recorded. - See the `overhead calculation' section for more details. Enabled by + See the `Overhead calculation' section for more details. Enabled by default, disable with --no-children. --max-stack:: @@ -447,9 +460,9 @@ OPTIONS --call-graph option for details. --percentage:: - Determine how to display the overhead percentage of filtered entries. - Filters can be applied by --comms, --dsos and/or --symbols options and - Zoom operations on the TUI (thread, dso, etc). + Determine how to display the CPU and latency overhead percentage + of filtered entries. Filters can be applied by --comms, --dsos, --symbols + and/or --parallelism options and Zoom operations on the TUI (thread, dso, etc). "relative" means it's relative to filtered entries only so that the sum of shown entries will be always 100%. "absolute" means it retains @@ -632,6 +645,8 @@ include::itrace.txt[] --skip-empty:: Do not print 0 results in the --stat output. +include::cpu-and-latency-overheads.txt[] + include::callchain-overhead-calculation.txt[] SEE ALSO diff --git a/tools/perf/Documentation/tips.txt b/tools/perf/Documentation/tips.txt index 67b326ba00407..3fee9b2a88ea9 100644 --- a/tools/perf/Documentation/tips.txt +++ b/tools/perf/Documentation/tips.txt @@ -62,3 +62,7 @@ To show context switches in perf report sample context add --switch-events to pe To show time in nanoseconds in record/report add --ns To compare hot regions in two workloads use perf record -b -o file ... ; perf diff --stream file1 file2 To compare scalability of two workload samples use perf diff -c ratio file1 file2 +For latency profiling, try: perf record/report --latency +For parallelism histogram, try: perf report --hierarchy --sort latency,parallelism,comm,symbol +To analyze particular parallelism levels, try: perf report --latency --parallelism=32-64 +To see how parallelism changes over time, try: perf report -F time,latency,parallelism --time-quantum=1s -- 2.48.1.502.g6dc24dfdaf-goog