DBサーバの空きメモリサイズの見方(Oracle Database on Linux)

JPOUG Advent Calendar 2015 の5日目のエントリーです。
昨日は [twitter:@discus_hamburg] さんの Mac De Oracle: OTHER_XMLの中身でした。

Linux で Oracle Database を使っている場合の”実質的な”空きメモリサイズの算出方法を紹介します。
絵は「シンプルでシステマチックなLinux性能分析方法」 @ db tech showcase 東京 2014 - ablog で使った資料から抜粋しています。

RHEL/Oracle Linux 5

空きメモリサイズ = /proc/meminfo の MemTotal - (
        vmstat の used (= free の used（-/+ buffers/cache)) … カーネル + プロセス
        + ipcs -um の pages resident * 4KB(ページサイズ、HugePages使用時は2MB) … System V IPC 共有メモリ
        + df -k の tmpfs の Used … tmpfs/ramfs (MEMORY_TARGET使用時)
    )

RHEL/Oracle Linux 6

空きメモリサイズ = /proc/meminfo の MemFree + Active(file) + Inactive(file)

free コマンドに追加された Available フィールド

Linux kernel 3.14 から /proc/meminfo に MemAvailable というフィールドが追加され、freeコマンドにもAvailableというフィールドが追加されています。RHEL7 や 6.6 にもバックポートされています(RHEL6.6 では互換性に配慮してデフォルトでは disable)。
これはスワップアウト*1なしでメモリ割当できるサイズで、めちゃざっくりでは以下の計算式になります。

MemAvailable = ( MemFree -
 wmark_
low ) + Active(file) + Inactive(file) + SReclaimable
# wmark_low 以外は /proc/meminfo のフィールド
# SReclaimable は回収可能なスラブキャッシュ
# wmark_low = vm.min_free_kbytes + ( vm.min_free_kbytes / 4 )

wmark_low については vm.min_free_kbytes からの wmark_{min|low|high} 算出式 - ablog をご覧ください。
以下は Linux kernel の git repository から /proc/meminfo に MemAvailable が追加された際の変更内容からの抜粋です。

/proc/meminfo: provide estimated available memory
Many load balancing and workload placing programs check /proc/meminfo to estimate how much free memory is available. They generally do this by adding up "free" and "cached", which was fine ten years ago, but is pretty much guaranteed to be wrong today.

It is wrong because Cached includes memory that is not freeable as page cache, for example shared memory segments, tmpfs, and ramfs, and it does not include reclaimable slab memory, which can take up a large fraction of system memory on mostly idle systems with lots of files.

Currently, the amount of memory that is available for a new workload, without pushing the system into swap, can be estimated from MemFree, Active(file), Inactive(file), and SReclaimable, as well as the "low"
watermarks from /proc/zoneinfo.

However, this may change in the future, and user space really should not be expected to know kernel internals to come up with an estimate for the amount of free memory.

It is more convenient to provide such an estimate in /proc/meminfo. If things change in the future, we only have to change it in one place.

Signed-off-by: Rik van Riel
Reported-by: Erik Mouw
Acked-by: Johannes Weiner
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds

2 files changed, 46 insertions, 0 deletions
diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 22d89aa3..8533f5f 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -767,6 +767,7 @@ The "Locked" indicates whether the mapping is locked in memory or not.
 
 MemTotal:     16344972 kB
 MemFree:      13634064 kB
+MemAvailable: 14836172 kB
 Buffers:          3656 kB
 Cached:        1195708 kB
 SwapCached:          0 kB
@@ -799,6 +800,14 @@ AnonHugePages:   49152 kB
     MemTotal: Total usable ram (i.e. physical ram minus a few reserved
               bits and the kernel binary code)
      MemFree: The sum of LowFree+HighFree
+MemAvailable: An estimate of how much memory is available for starting new
+              applications, without swapping. Calculated from MemFree,
+              SReclaimable, the size of the file LRU lists, and the low
+              watermarks in each zone.
+              The estimate takes into account that the system needs some
+              page cache to function well, and that not all reclaimable
+              slab will be reclaimable, due to items being in use. The
+              impact of those factors will vary from system to system.
      Buffers: Relatively temporary storage for raw disk blocks
               shouldn't get tremendously large (20MB or so)
       Cached: in-memory cache for files read from the disk (the
diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
index a77d2b2..24270ec 100644
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -26,7 +26,11 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	unsigned long committed;
 	struct vmalloc_info vmi;
 	long cached;
+	long available;
+	unsigned long pagecache;
+	unsigned long wmark_low = 0;
 	unsigned long pages[NR_LRU_LISTS];
+	struct zone *zone;
 	int lru;
 
 /*
@@ -47,12 +51,44 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 	for (lru = LRU_BASE; lru < NR_LRU_LISTS; lru++)
 		pages[lru] = global_page_state(NR_LRU_BASE + lru);
 
+	for_each_zone(zone)
+		wmark_low += zone->watermark[WMARK_LOW];
+
+	/*
+	 * Estimate the amount of memory available for userspace allocations,
+	 * without causing swapping.
+	 *
+	 * Free memory cannot be taken below the low watermark, before the
+	 * system starts swapping.
+	 */
+	available = i.freeram - wmark_low;
+
+	/*
+	 * Not all the page cache can be freed, otherwise the system will
+	 * start swapping. Assume at least half of the page cache, or the
+	 * low watermark worth of cache, needs to stay.
+	 */
+	pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];
+	pagecache -= min(pagecache / 2, wmark_low);
+	available += pagecache;
+
+	/*
+	 * Part of the reclaimable swap consists of items that are in use,
+	 * and cannot be freed. Cap this estimate at the low watermark.
+	 */
+	available += global_page_state(NR_SLAB_RECLAIMABLE) -
+		     min(global_page_state(NR_SLAB_RECLAIMABLE) / 2, wmark_low);
+
+	if (available < 0)
+		available = 0;
+
 	/*
 	 * Tagged format, for easy grepping and expansion.
 	 */
 	seq_printf(m,
 		"MemTotal:       %8lu kB\n"
 		"MemFree:        %8lu kB\n"
+		"MemAvailable:   %8lu kB\n"
 		"Buffers:        %8lu kB\n"
 		"Cached:         %8lu kB\n"
 		"SwapCached:     %8lu kB\n"
@@ -105,6 +141,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 		,
 		K(i.totalram),
 		K(i.freeram),
+		K(available),
 		K(i.bufferram),
 		K(cached),
 		K(total_swapcache_pages()),

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773

さらに詳しく知りたい方はのぴぴさんの以下のエントリをご覧ください。

明日6日目は [twitter:@s4r_agent] さん(ブログはコチラ)です。

参考

*1:正確にはページアウト

ablog

不器用で落着きのない技術者のメモ

DBサーバの空きメモリサイズの見方(Oracle Database on Linux)

RHEL/Oracle Linux 5

RHEL/Oracle Linux 6

RHEL/Oracle Linux 7

free コマンドに追加された Available フィールド

関連

JPOUG Advent Calendar

メモリ関連

その他

参考