LIBPFM(3)

💖 LIBPFM(3) — Linux Programmer's Manual 💖

okay so bestie, welcome to the libpfm_intel_snbep_unc_cbo page from the Linux Programmer's Manual 🫶 this is the one about Intel Sandy Bridge-EP C-Box uncore PMU support, and ngl it's kinda niche but it's also kinda iconic. sit down, grab a strawberry 🍓, let's get into it.

✨ NAME ✨

libpfm_intel_snbep_unc_cbo — support for Intel Sandy Bridge-EP C-Box uncore PMU

like. that's the whole vibe. it's the support page. 💅

🌈 SYNOPSIS 🌈

#include <perfmon/pfmlib.h>

PMU name: snbep_unc_cbo[0-7]
PMU desc: Intel Sandy Bridge-EP C-Box uncore PMU

so you pull in perfmon/pfmlib.h, and the PMU you're working with is named snbep_unc_cbo[0-7] and its description is literally "Intel Sandy Bridge-EP C-Box uncore PMU". that's it. that's the whole intro. 💜

🦋 DESCRIPTION 🦋

so this library supports the Intel Sandy Bridge C-Box (coherency engine) uncore PMU. periodt.

heads up: this PMU model only exists on Sandy Bridge model 45. no other model. just 45. don't come for me, that's what it says 💀

there is one C-box PMU per physical core. so do the math bestie, that means there are eight identical C-Box PMU instances, numbered from 0 to 7. eight. one per core. slay.

now here's the tea on dual-socket systems ☕ — the number refers to the C-Box PMU on the socket where the program runs. so like, if you're running on CPU8, then snbep_unc_cbo0 refers to the C-Box for physical core 0 on socket 1. but flip it — if you're running on CPU0, then that same snbep_unc_cbo0 refers to the C-Box for physical core 0 but on socket 0. same name, different socket, depending on where you're standing. it's giving relativity honestly 🧚

each C-Box PMU implements 4 generic counters and a filter register that's used only with certain events and umasks. lowkey important detail, don't skip it.

👑 MODIFIERS 👑

okay this is the main character section. the following modifiers are supported on the Intel Sandy Bridge C-Box uncore PMU:

• i — invert the meaning of the event 💅 the counter will now count C-Box cycles in which the event is not occurring. this is 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 threshold modifier (t) with a value greater or equal to one. this is a boolean modifier.
• t — set the threshold value. when set to a non-zero value, the counter counts the number of C-Box 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]. no cap on the drama, but there is a cap on the value 🔥
• nf — node filter 🌸 certain events, such as UNC_C_LLC_LOOKUP, UNC_C_LLC_VICTIMS, provide a NID umask. sometimes the NID is combined with other filtering capabilities, such as opcodes. the node filter is an 8-bit max bitmask. a node corresponds to a processor socket. the legal values therefore depend on the underlying hardware configuration. for dual-socket systems, the bitmask has two valid bits [0:1].
• cf — Core Filter 💖 this is a 3-bit filter used to filter based on the physical core origin of the C-Box request. possible values are 0-7. if the filter is not specified, then no filtering takes place. simple as.
• tf — Thread Filter 🎀 this is a 1-bit filter used to filter C-Box requests based on logical processor (hyper-thread) identification. possible values are 0-1. if the filter is not specified, then no filtering takes place. same energy as above tbh.

🔍 Opcode filtering 🔍

certain events, such as UNC_C_TOR_INSERTS, support opcode matching on the C-BOX transaction type. here's how you do it, step by step bestie:

1. first, an opcode matching umask must be selected, e.g., MISS_OPCODE.
2. second, the opcode to match on must be selected via a second umask among the OPC_* umasks.

for instance, UNC_C_TOR_INSERTS:OPCODE:OPC_RFO counts the number of TOR insertions for RFO transactions. ate. ✨

opcode matching may be combined with node filtering with certain umasks. in general the filtering support is encoded into the umask name, e.g., NID_OPCODE supports both node and opcode filtering. for instance:

UNC_C_TOR_INSERTS:NID_OPCODE:OPC_RFO:nf=1

iykyk 💅

🫶 AUTHORS 🫶

Stephane Eranian <eranian@gmail.com>

and that's the whole page, bestie 💖 this was the LIBPFM(3) page from August, 2012, part of the Linux Programmer's Manual — no cap, that's genuinely the source and date. go forth and count your C-Box cycles responsibly 🦋✨🎀