一、需求与背景
LINUX主机上的内存很为user、buffer、cache这个大的块。经常在使用free -m查看时,会发现cache占去了大头,从理论上我们也一直都知道cache里存放的是系统上访问的热数据(不经常访问的数据会被标记后,更换为新的数据),这样会加快主机的访问速度。而cache内部这些数据的命中率如何查看呢?今天在跟相关人员分析现网一个问题时,有同事提到的一个问题。
二、cache命中率计算方法
网上找了下Brendan Gregg 在 perf-tools 里集成了一个工具cachestat,可以查看cache命中率,该工具是用shell 写的,读了下源码,发现其是通过读取/sys/kernel/debug/tracing/trace_stat/ 目录下的所有function调用,查看其中的四项内容:
mark_page_accessed() for measuring cache accesses
mark_buffer_dirty() for measuring cache writes
add_to_page_cache_lru() for measuring page additions
account_page_dirtied() for measuring page dirties
而四者之间的值是按如下公式计算的:
总cache page数(total) = mark_page_accessed(标记为访问的) - mark_buffer_dirty(标记为脏的)
misses = add_to_page_cache_lru(新增的) - account_page_dirtied (统计为脏状态的)
total - misses = hit
hit/total = ratio (命中率)
这里给一个读取人trace_stat下的文件的内容,如下:
在该工具同时还读取了/proc/meminfo,不过读取该文件并不参与命中率计算,读取这个文件的内容只是用于算出目前使用的cache大小,并在界面输出。
三、cachestat代码及执行
cachestat代码如下:
#!/bin/bash
# 28-Dec-2014 Brendan Gregg Created this.
### default variables
tracing=/sys/kernel/debug/tracing
interval=1; opt_timestamp=0; opt_debug=0
trap 'quit=1' INT QUIT TERM PIPE HUP # sends execution to end tracing section
function usage {
cat &2
USAGE: cachestat [-Dht] [interval]
-D # print debug counters
-h # this usage message
-t # include timestamp
interval # output interval in secs (default 1)
eg,
cachestat # show stats every second
cachestat 5 # show stats every 5 seconds
See the man page and example file for more info.
END
exit
}
function warn {
if ! eval "$@"; then
echo >&2 "WARNING: command failed \"$@\""
fi
}
function die {
echo >&2 "$@"
exit 1
}
### process options
while getopts Dht opt
do
case $opt in
D) opt_debug=1 ;;
t) opt_timestamp=1 ;;
h|?) usage ;;
esac
done
shift $(( $OPTIND - 1 ))
### option logic
if (( $# )); then
interval=$1
fi
echo "Counting cache functions... Output every $interval seconds."
### check permissions
cd $tracing || die "ERROR: accessing tracing. Root user? Kernel has FTRACE?
debugfs mounted? (mount -t debugfs debugfs /sys/kernel/debug)"
### enable tracing
sysctl -q kernel.ftrace_enabled=1 # doesn't set exit status
printf "mark_page_accessed\nmark_buffer_dirty\nadd_to_page_cache_lru\naccount_page_dirtied\n" > set_ftrace_filter || \
die "ERROR: tracing these four kernel functions: mark_page_accessed,"\
"mark_buffer_dirty, add_to_page_cache_lru and account_page_dirtied (unknown kernel version?). Exiting."
warn "echo nop > current_tracer"
if ! echo 1 > function_profile_enabled; then
echo > set_ftrace_filter
die "ERROR: enabling function profiling. Have CONFIG_FUNCTION_PROFILER? Exiting."
fi
(( opt_timestamp )) && printf "%-8s " TIME
printf "%8s %8s %8s %8s %12s %10s" HITS MISSES DIRTIES RATIO "BUFFERS_MB" "CACHE_MB"
(( opt_debug )) && printf " DEBUG"
echo
### summarize
quit=0; secs=0
while (( !quit && (!opt_duration || secs function_profile_enabled
echo 1 > function_profile_enabled
sleep $interval
(( opt_timestamp )) && printf "%(%H:%M:%S)T " -1
# cat both meminfo and trace stats, and let awk pick them apart
cat /proc/meminfo trace_stat/function* | awk -v debug=$opt_debug '
# match meminfo stats:
$1 == "Buffers:" && $3 == "kB" { buffers_mb = $2 / 1024 }
$1 == "Cached:" && $3 == "kB" { cached_mb = $2 / 1024 }
# identify and save trace counts:
$2 ~ /[0-9]/ && $3 != "kB" { a[$1] += $2 }
END {
mpa = a["mark_page_accessed"]
mbd = a["mark_buffer_dirty"]
apcl = a["add_to_page_cache_lru"]
apd = a["account_page_dirtied"]
total = mpa - mbd
misses = apcl - apd
if (misses /dev/null
echo "Ending tracing..." 2>/dev/null
warn "echo 0 > function_profile_enabled"
warn "echo > set_ftrace_filter"
脚本执行后,结果如下:
参考页面:
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