LIBPFM(3)

💖 LIBPFM 💖

okay bestie, sit down, hydrate, because we're talking about the Linux Programmer's Manual page for libpfm, and honestly? it's giving niche-but-iconic. ✨

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✨ NAME ✨

libpfm_intel_bdw — support for Intel Broadwell core PMU 🎀

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🌸 SYNOPSIS 🌸

#include <perfmon/pfmlib.h>

PMU name: bdw
PMU desc: Intel Broadwell

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💅 DESCRIPTION 💅

So the library supports the Intel Broadwell core PMU, and lowkey you need to know one thing before anything else: this PMU model only covers each core's PMU, not the socket level PMU. like, core-level only, no cap. 🙈

Now, on Broadwell, the number of generic counters is dependent on the Hyperthreading (HT) mode, and this is the part where the vibe check actually matters:

• When HT is on, only 4 generic counters are available. 💀
• When HT is off, 8 generic counters are available. 👑

And the pfm_get_pmu_info() function returns the maximum number of generic counters in num_cntrs. so that's your little lookup bestie for figuring out how many you actually get. 🫶

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✨ MODIFIERS ✨

The following modifiers are supported on Intel Broadwell processors:

• u — measure at user level, which includes privilege levels 1, 2, 3. This corresponds to PFM_PLM3. It's a boolean modifier. 💖
• k — measure at kernel level, which includes privilege level 0. This corresponds to PFM_PLM0. It's a boolean modifier. 🦋
• i — invert the meaning of the event. The counter will now count cycles in which the event is not occurring. It's a boolean modifier. 🔄
• e — enable edge detection, i.e., count only when there's a state transition from no occurrence of the event to at least one occurrence. This modifier must be combined with a counter mask modifier (m) with a value greater or equal to one. It's a boolean modifier. ⚡
• c — set the counter mask value. The mask acts as a threshold: the counter will count the number of cycles in which the number of occurrences of the event is greater or equal to the threshold. This is an integer modifier with values in the range [0:255]. 🔢
• t — measure on both threads at the same time, assuming hyper-threading is enabled. It's a boolean modifier. 🧵
• ldlat — pass a latency threshold to the MEM_TRANS_RETIRED:LOAD_LATENCY event. This is an integer attribute that must be in the range [1:65535]. It is required for this event. Note that the event must be used with precise sampling (PEBS). ⏱️
• intx — monitor the event only when executing inside a transactional memory region (in tx). The event does not count otherwise. It's a boolean modifier. Default value is 0. 🌈
• intxcp — do not count occurrences of the event when they are inside an aborted transactional memory region. It's a boolean modifier. Default value is 0. 🙅♀️

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🔥 OFFCORE_RESPONSE events 🔥

Intel Broadwell provides two offcore_response events. They are called OFFCORE_RESPONSE_0 and OFFCORE_RESPONSE_1. periodt.

Those events need special treatment in the performance monitoring infrastructure, because each event uses an extra register</> to store some settings. So in case multiple offcore_response events are monitored simultaneously, the kernel needs to manage the sharing of that extra register. teamwork, basically. 🤝

The offcore_response events are exposed as normal events by the library, and the extra settings are exposed as regular umasks. The library takes care of encoding the events according to the underlying kernel interface. so you don't have to do it yourself, slay. 💅

On Intel Broadwell, the umasks are divided into three categories: request, supplier and snoop. The user must provide at least one umask for each category. The categories are shown in the umask descriptions. 👀

There's also the special response umask called ANY_RESPONSE. When this umask is used, then it overrides any supplier and snoop umasks. In other words, users can specify either ANY_RESPONSE or any combinations of supplier + snoops. pick a lane bestie, you can't do both. 😭

In case no supplier or snoop is specified, the library defaults to using ANY_RESPONSE.

For instance, the following are valid event selections:

OFFCORE_RESPONSE_0:DMND_DATA_RD:ANY_RESPONSE

OFFCORE_RESPONSE_0:ANY_REQUEST

OFFCORE_RESPONSE_0:ANY_RFO:LLC_HITM:SNOOP_ANY

But the following are illegal:

OFFCORE_RESPONSE_0:ANY_RFO:LLC_HITM:ANY_RESPONSE

OFFCORE_RESPONSE_0:ANY_RFO:LLC_HITM:SNOOP_ANY:ANY_RESPONSE

🚨 bestie warning: yeah, that second list is a whole no-no. don't mix ANY_RESPONSE in with your supplier + snoop combo, the library will not be happy with you.

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🫶 AUTHORS 🫶

Stephane Eranian <eranian@gmail.com> 💜

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and that's the whole page, bestie!! written back in October, 2014, and it lives in the LIBPFM(3) section of the manual. iconic, timeless, still relevant. go forth and count those cycles. 👑✨🎀