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| | Very nice site! | | Very nice site! |
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| − | = How can you use this =
| + | Hello! aaeebda interesting aaeebda site! I'm really like it! Very, very aaeebda good! |
| − | | |
| − | Imagine, you are doing a file system read benchmark. Your system is fresh:
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| − | tweedleburg:~ # free | |
| − | total used free shared buffers cached
| |
| − | Mem: 4053216 795664 3257552 0 352 '''54624'''
| |
| − | -/+ buffers/cache: 740688 3312528
| |
| − | Swap: 0 0 0
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| − | You have 54624 bytes in all caches.
| |
| − | tweedleburg:~ # dd if=wine-1.0-rc2.tar of=/dev/null
| |
| − | 197360+0 records in
| |
| − | 197360+0 records out
| |
| − | 101048320 bytes (101 MB) copied, 2.278 s, '''44.4 MB/s'''
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| − | You get 44.4 MB/s for disk reads, a realistic result.
| |
| − | tweedleburg:~ # dd if=wine-1.0-rc2.tar of=/dev/null
| |
| − | 197360+0 records in
| |
| − | 197360+0 records out
| |
| − | 101048320 bytes (101 MB) copied, 0.190445 s, '''531 MB/s'''
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| − | At the second time, you get 531 MB/s for disk reads, an unrealistically good result. The culprit are the caches that stored the file content:
| |
| − | tweedleburg:~ # free
| |
| − | total used free shared buffers cached
| |
| − | Mem: 4053216 886360 3166856 0 528 '''145748'''
| |
| − | -/+ buffers/cache: 740084 3313132
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| − | Swap: 0 0 0
| |
| − | You want to clear the caches.
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| − | | |
| − | '''Note:''' To drop your caches, you can also use [http://linux-mm.org/Drop_Caches /proc/sys/vm/drop_caches]
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| − | | |
| − | You use my program to clear the caches:
| |
| − | tweedleburg:~ # ./a.out >/dev/null
| |
| − | terminate called after throwing an instance of 'St9bad_alloc'
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| − | what(): std::bad_alloc
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| − | Aborted
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| − | My program allocates memory till it can no more. All caches are eaten up. Then it stops and frees its memory:
| |
| − | tweedleburg:~ # free
| |
| − | total used free shared buffers cached
| |
| − | Mem: 4053216 794916 3258300 0 344 '''57360'''
| |
| − | -/+ buffers/cache: 737212 3316004
| |
| − | Swap: 0 0 0
| |
| − | You repeat the file read:
| |
| − | tweedleburg:~ # dd if=wine-1.0-rc2.tar of=/dev/null
| |
| − | 197360+0 records in
| |
| − | 197360+0 records out
| |
| − | 101048320 bytes (101 MB) copied, 2.21617 s, '''45.6 MB/s'''
| |
| − | and get a realistic result. To show you I am not telling bullshit, we repeat the disk read without cleaning the caches and get again an unrealistic result:
| |
| − | tweedleburg:~ # dd if=wine-1.0-rc2.tar of=/dev/null
| |
| − | 197360+0 records in
| |
| − | 197360+0 records out
| |
| − | 101048320 bytes (101 MB) copied, 0.165862 s, '''609 MB/s'''
| |
| − | You see there is a big difference between a cached and a non-cached read; but also there is a difference to a direct read:
| |
| − | tweedleburg:~ # dd iflag=direct if=wine-1.0-rc2.tar of=/dev/null
| |
| − | 197360+0 records in
| |
| − | 197360+0 records out
| |
| − | 101048320 bytes (101 MB) copied, 23.6831 s, '''4.3 MB/s'''
| |
| − | reading with [[dd]] using the direct flag yields the same [[performance]] as reading with an empty cache, you just have to adjust the blocksize:
| |
| − | tweedleburg:~ # dd iflag=direct if=wine-1.0-rc2.tar bs=1024k of=/dev/null
| |
| − | 96+1 records in
| |
| − | 96+1 records out
| |
| − | 101048320 bytes (101 MB) copied, 2.42799 s, '''41.6 MB/s'''
| |
You know that? The longer you work with your Linux system, the bigger the memory consumption is. However, performance is not negatively affected. Looking closely at it, you see only the cache has grown bigger. The article writes a program that eats up all memory, then terminates with a std::bad_alloc. Before terminating, it has of course eaten up all your cache and transformed into usr memory. Then, the memory is freed and there is no more cache.
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