linux – 30％的RAM是“缓冲区”.它是什么？

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$free -h
              total        used        free      shared  buff/cache   available
Mem:           501M        146M         19M        9.7M        335M        331M
Swap:          1.0G         85M        938M

$free -w -h
              total        used        free      shared     buffers       cache   available
Mem:           501M        146M         19M        9.7M        155M        180M        331M
Swap:          1.0G         85M        938M

如何在免费输出中描述或解释“缓冲区”？

我对这个系统没有任何(已知的)问题.我很惊讶并且好奇地看到“缓冲区”几乎和“缓存”一样高(155M v.s.180M).我认为“缓存”代表文件内容的页面缓存,并且往往是“缓存/缓冲区”中最重要的部分.我不太清楚什么是“缓冲”.

例如,我把它与我的笔记本电脑进行了比较在我的笔记本电脑上,“缓冲”数字比“缓存”(200M v.s. 4G)小一个数量级.如果我对“缓冲区”有了正确的理解,那么我就可以开始问为什么缓冲区可能会在较小的系统上增长到如此大的比例.

man proc(我忽略了“大”的滑稽过时的定义)：

Buffers %lu

Relatively temporary storage for raw disk blocks that shouldn’t get tremendously large (20MB or so).

Cached %lu

In-memory cache for files read from the disk (the page cache). Doesn’t include SwapCached.

$free -V
free from procps-ng 3.3.12
$uname -r
4.9.0-6-marvell
$systemd-detect-virt
none

$cat /proc/meminfo
MemTotal:         513976 kB
MemFree:           20100 kB
MemAvailable:     339304 kB
Buffers:          159220 kB
Cached:           155536 kB
SwapCached:         2420 kB
Active:           215044 kB
Inactive:         216760 kB
Active(anon):      56556 kB
Inactive(anon):    73280 kB
Active(file):     158488 kB
Inactive(file):   143480 kB
Unevictable:       10760 kB
Mlocked:           10760 kB
HighTotal:             0 kB
HighFree:              0 kB
LowTotal:         513976 kB
LowFree:           20100 kB
SwapTotal:       1048572 kB
SwapFree:         960532 kB
Dirty:               240 kB
Writeback:             0 kB
AnonPages:        126912 kB
Mapped:            40312 kB
Shmem:              9916 kB
Slab:              37580 kB
SReclaimable:      29036 kB
SUnreclaim:         8544 kB
KernelStack:        1472 kB
PageTables:         3108 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     1305560 kB
Committed_AS:    1155244 kB
VmallocTotal:     507904 kB
VmallocUsed:           0 kB
VmallocChunk:          0 kB

$sudo slabtop --once
 Active / Total Objects (% used)    : 186139 / 212611 (87.5%)
 Active / Total Slabs (% used)      : 9115 / 9115 (100.0%)
 Active / Total Caches (% used)     : 66 / 92 (71.7%)
 Active / Total Size (% used)       : 31838.34K / 35031.49K (90.9%)
 Minimum / Average / Maximum Object : 0.02K / 0.16K / 4096.00K

  OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME                   
 59968  57222   0%    0.06K    937       64      3748K buffer_head            
 29010  21923   0%    0.13K    967       30      3868K dentry                 
 24306  23842   0%    0.58K   4051        6     16204K ext4_inode_cache       
 22072  20576   0%    0.03K    178      124       712K kmalloc-32             
 10290   9756   0%    0.09K    245       42       980K kmalloc-96             
  9152   4582   0%    0.06K    143       64       572K kmalloc-node           
  9027   8914   0%    0.08K    177       51       708K kernfs_node_cache      
  7007   3830   0%    0.30K    539       13      2156K radix_tree_node        
  5952   4466   0%    0.03K     48      124       192K jbd2_revoke_record_s   
  5889   5870   0%    0.30K    453       13      1812K inode_cache            
  5705   4479   0%    0.02K     35      163       140K file_lock_ctx          
  3844   3464   0%    0.03K     31      124       124K anon_vma               
  3280   3032   0%    0.25K    205       16       820K kmalloc-256            
  2730   2720   0%    0.10K     70       39       280K btrfs_trans_handle     
  2025   1749   0%    0.16K     81       25       324K filp                   
  1952   1844   0%    0.12K     61       32       244K kmalloc-128            
  1826    532   0%    0.05K     22       83        88K trace_event_file       
  1392   1384   0%    0.33K    116       12       464K proc_inode_cache       
  1067   1050   0%    0.34K     97       11       388K shmem_inode_cache      
   987    768   0%    0.19K     47       21       188K kmalloc-192            
   848    757   0%    0.50K    106        8       424K kmalloc-512            
   450    448   0%    0.38K     45       10       180K ubifs_inode_slab       
   297    200   0%    0.04K      3       99        12K eventpoll_pwq          
   288    288 100%    1.00K     72        4       288K kmalloc-1024           
   288    288 100%    0.22K     16       18        64K mnt_cache              
   287    283   0%    1.05K     41        7       328K idr_layer_cache        
   240      8   0%    0.02K      1      240         4K fscrypt_info           

解决方法:

>“缓冲区”和其他缓存有什么区别？
> Linux何时使用缓冲区缓存而不是页面缓存？
>为什么我们可以期望缓冲区缓存更大或更小？

1.“缓冲区”和其他缓存有什么区别？

可以肯定地说缓冲区缓存是“原始磁盘块的存储”.文件系统使用的磁盘块可能小至512字节.这与用于文件数据的页面缓存不同.

页面缓存与MMU页面大小相关联,通常最小为4096字节.这对于存储器映射文件访问[2]是必不可少的,并且具体地用于按需加载运行程序的各个代码页. (另外,当其他东西需要空间时卸载页面,并且最近没有使用它们).

2. Linux什么时候使用缓冲区缓存而不是页面缓存？

在现代Linux上,缓冲区缓存很可能由文件系统元数据组成.这包括目录内容.

缓冲区缓存的这种使用取决于文件系统,但它包括有问题的系统使用的ext4.

Buffers [in /proc/meminfo] are in-memory block I/O buffers. They are relatively short-lived. Prior to Linux kernel version 2.4, Linux had separate page and buffer caches. Since 2.4, the page and buffer cache are unified and Buffers is raw disk blocks not represented in the page cache—i.e., not file data.

…

The buffer cache remains, however, as the kernel still needs to perform block I/O in terms of blocks, not pages. As most blocks represent file data, most of the buffer cache is represented by the page cache. But a small amount of block data isn’t file backed—metadata and raw block I/O for example—and thus is solely represented by the buffer cache.

– Robert Love by Robert Love,最后更新2013.

Certain file systems, including ext3, ext4, and ocfs2, use the jbd or
jbd2 layer to handle their physical block journalling, and this layer
fundamentally uses the buffer cache, since it is concerned with
controlling when specific file system blocks are allowed to ben
written back to the hard drive.

Other file systems may not support file system blocks smaller than 4k.
This may make it easier for them to use the page cache for their
metadata blocks, although I don’t know what happens if you try to
mount a btrfs file system formatted with 4k blocks on an architecture
such as Power which has 16k pages. I don’t know if it will work, or
blow up in a spectacular display of sparks.

标签：,, 来源： https://codeday.me/bug/20190809/1630831.html

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