perf-c2c - Shared Data C2C/HITM Analyzer.
perf c2c record [<options>] <command>
perf c2c record [<options>] -- [<record command options>] <command>
perf c2c report [<options>]C2C stands for Cache To Cache.
The perf c2c tool provides means for Shared Data C2C/HITM analysis. It allows you to track down the cacheline contentions.
On x86, the tool is based on load latency and precise store facility events provided by Intel CPUs. On PowerPC, the tool uses random instruction sampling with thresholding feature.
These events provide: - memory address of the access - type of the access (load and store details) - latency (in cycles) of the load access
The c2c tool provide means to record this data and report back access details for cachelines with highest contention - highest number of HITM accesses.
The basic workflow with this tool follows the standard record/report phase. User uses the record command to record events data and report command to display it.
-e, --event=
Select the PMU event. Use perf c2c record -e list to list available events.
-v, --verbose
Be more verbose (show counter open errors, etc).
-l, --ldlat
Configure mem-loads latency. (x86 only)
-k, --all-kernel
Configure all used events to run in kernel space.
-u, --all-user
Configure all used events to run in user space.
-k, --vmlinux=<file>
vmlinux pathname
-v, --verbose
Be more verbose (show counter open errors, etc).
-i, --input
Specify the input file to process.
-N, --node-info
Show extra node info in report (see NODE INFO section)
-c, --coalesce
Specify sorting fields for single cacheline display. Following fields are available: tid,pid,iaddr,dso (see COALESCE)
-g, --call-graph
Setup callchains parameters. Please refer to perf-report man page for details.
--stdio
Force the stdio output (see STDIO OUTPUT)
--stats
Display only statistic tables and force stdio mode.
--full-symbols
Display full length of symbols.
--no-source
Do not display Source:Line column.
--show-all
Show all captured HITM lines, with no regard to HITM % 0.0005 limit.
-f, --force
Don’t do ownership validation.
-d, --display
Switch to HITM type (rmt, lcl) to display and sort on. Total HITMs as default.
--stitch-lbr
Show callgraph with stitched LBRs, which may have more complete callgraph. The perf.data file must have been obtained using perf c2c record --call-graph lbr. Disabled by default. In common cases with call stack overflows, it can recreate better call stacks than the default lbr call stack output. But this approach is not full proof. There can be cases where it creates incorrect call stacks from incorrect matches. The known limitations include exception handing such as setjmp/longjmp will have calls/returns not match.
The perf c2c record command setup options related to HITM cacheline analysis and calls standard perf record command.
Following perf record options are configured by default: (check perf record man page for details)
-W,-d,--phys-data,--sample-cpu
Unless specified otherwise with -e option, following events are monitored by default on x86:
cpu/mem-loads,ldlat=30/P cpu/mem-stores/P
and following on PowerPC:
cpu/mem-loads/ cpu/mem-stores/
User can pass any perf record option behind -- mark, like (to enable callchains and system wide monitoring):
$ perf c2c record -- -g -a
Please check RECORD OPTIONS section for specific c2c record options.
The perf c2c report command displays shared data analysis. It comes in two display modes: stdio and tui (default).
The report command workflow is following: - sort all the data based on the cacheline address - store access details for each cacheline - sort all cachelines based on user settings - display data
In general perf report output consist of 2 basic views: 1) most expensive cachelines list 2) offsets details for each cacheline
For each cacheline in the 1) list we display following data: (Both stdio and TUI modes follow the same fields output)
Index - zero based index to identify the cacheline
Cacheline - cacheline address (hex number)
Rmt/Lcl Hitm - cacheline percentage of all Remote/Local HITM accesses
LLC Load Hitm - Total, LclHitm, RmtHitm - count of Total/Local/Remote load HITMs
Total records - sum of all cachelines accesses
Total loads - sum of all load accesses
Total stores - sum of all store accesses
Store Reference - L1Hit, L1Miss L1Hit - store accesses that hit L1 L1Miss - store accesses that missed L1
Core Load Hit - FB, L1, L2 - count of load hits in FB (Fill Buffer), L1 and L2 cache
LLC Load Hit - LlcHit, LclHitm - count of LLC load accesses, includes LLC hits and LLC HITMs
RMT Load Hit - RmtHit, RmtHitm - count of remote load accesses, includes remote hits and remote HITMs
Load Dram - Lcl, Rmt - count of local and remote DRAM accesses
For each offset in the 2) list we display following data:
HITM - Rmt, Lcl - % of Remote/Local HITM accesses for given offset within cacheline
Store Refs - L1 Hit, L1 Miss - % of store accesses that hit/missed L1 for given offset within cacheline
Data address - Offset - offset address
Pid - pid of the process responsible for the accesses
Tid - tid of the process responsible for the accesses
Code address - code address responsible for the accesses
cycles - rmt hitm, lcl hitm, load - sum of cycles for given accesses - Remote/Local HITM and generic load
cpu cnt - number of cpus that participated on the access
Symbol - code symbol related to the Code address value
Shared Object - shared object name related to the Code address value
Source:Line - source information related to the Code address value
Node - nodes participating on the access (see NODE INFO section)
The Node field displays nodes that accesses given cacheline offset. Its output comes in 3 flavors: - node IDs separated by , - node IDs with stats for each ID, in following format: Node{cpus %hitms %stores} - node IDs with list of affected CPUs in following format: Node{cpu list}
User can switch between above flavors with -N option or use n key to interactively switch in TUI mode.
User can specify how to sort offsets for cacheline.
Following fields are available and governs the final output fields set for cacheline offsets output:
tid - coalesced by process TIDs pid - coalesced by process PIDs iaddr - coalesced by code address, following fields are displayed: Code address, Code symbol, Shared Object, Source line dso - coalesced by shared object
By default the coalescing is setup with pid,iaddr.
The stdio output displays data on standard output.
Following tables are displayed: Trace Event Information - overall statistics of memory accesses
Global Shared Cache Line Event Information - overall statistics on shared cachelines
Shared Data Cache Line Table - list of most expensive cachelines
Shared Cache Line Distribution Pareto - list of all accessed offsets for each cacheline
The TUI output provides interactive interface to navigate through cachelines list and to display offset details.
For details please refer to the help window by pressing ? key.
Although Don Zickus, Dick Fowles and Joe Mario worked together to get this implemented, we got lots of early help from Arnaldo Carvalho de Melo, Stephane Eranian, Jiri Olsa and Andi Kleen.
Check Joe’s blog on c2c tool for detailed use case explanation: https://joemario.github.io/blog/2016/09/01/c2c-blog/