Linux cache命中率查看

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 <<-END >&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 < duration) )); do
	(( secs += interval ))
	echo 0 > 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 < 0)
			misses = 0
		hits = total - misses
		ratio = 100 * hits / total
		printf "%8d %8d %8d %7.1f%% %12.0f %10.0f", hits, misses, mbd,
		    ratio, buffers_mb, cached_mb
		if (debug)
			printf "  (%d %d %d %d)", mpa, mbd, apcl, apd
		printf "\n"
	}'
done
### end tracing
echo 2>/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：

http://www.brendangregg.com/blog/2014-12-31/linux-page-cache-hit-ratio.html （Brendan Gregg大神的博客）