There are several files in /proc/sys/vm you can use to tune the
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There are several files in /proc/sys/vm you can use to tune the
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memory system with.
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memory system with.
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You inspect them with 'cat', and set them with 'echo'. For example,
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You inspect them with 'cat', and set them with 'echo'. For example,
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/proc/sys/vm/freepages:
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/proc/sys/vm/freepages:
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'# cat /proc/sys/vm/freepages' may yield:
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'# cat /proc/sys/vm/freepages' may yield:
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64 96 128
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64 96 128
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These three numbers are: min_free_pages, free_pages_low and
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These three numbers are: min_free_pages, free_pages_low and
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free_pages_high.
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free_pages_high.
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You can adjust these with a command such as:
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You can adjust these with a command such as:
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# echo "128 256 512" > /proc/sys/vm/freepages
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# echo "128 256 512" > /proc/sys/vm/freepages
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Free memory never goes down below min_free_pages except for atomic
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Free memory never goes down below min_free_pages except for atomic
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allocation. Background swapping is started if the number of free
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allocation. Background swapping is started if the number of free
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pages falls below free_pages_high, and intensive swapping is started
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pages falls below free_pages_high, and intensive swapping is started
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below free_pages_low. A "page" is 4 kB.
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below free_pages_low. A "page" is 4 kB.
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The values selected as boot defaults are the following: For a
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The values selected as boot defaults are the following: For a
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machine with n>=8 Megabytes of memory, set min_free_pages = n*2,
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machine with n>=8 Megabytes of memory, set min_free_pages = n*2,
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free_pages_low = n*3 and free_pages_high = n*4. Machines with
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free_pages_low = n*3 and free_pages_high = n*4. Machines with
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8 Megabytes or less behave as if they had 8 Megabytes.
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8 Megabytes or less behave as if they had 8 Megabytes.
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If "out of memory" errors sometimes occur, or if your machine does lots
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If "out of memory" errors sometimes occur, or if your machine does lots
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of networking, increasing min_free_pages to 64 or more may be a good
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of networking, increasing min_free_pages to 64 or more may be a good
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idea.
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idea.
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free_pages_low should probably be about double of min_free_pages.
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free_pages_low should probably be about double of min_free_pages.
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After a period of inactivity, the difference between free_pages_high and
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After a period of inactivity, the difference between free_pages_high and
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free_pages low is immediately available for any program you want to
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free_pages low is immediately available for any program you want to
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start up, without any need to swap out anything else. If your memory
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start up, without any need to swap out anything else. If your memory
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is large enough (e.g. > 16 Meg), keeping 2 or 3 megabytes of memory
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is large enough (e.g. > 16 Meg), keeping 2 or 3 megabytes of memory
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ready for this purpose is probably a good idea.
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ready for this purpose is probably a good idea.
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I've found that
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I've found that
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# echo "128 256 1024" > /proc/sys/vm/freepages
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# echo "128 256 1024" > /proc/sys/vm/freepages
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gives good performance for a 32 Meg system used as a small server and
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gives good performance for a 32 Meg system used as a small server and
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personal workstation.
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personal workstation.
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The other three files in /proc/sys/vm are undocumented, as yet.
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The other three files in /proc/sys/vm are undocumented, as yet.
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Thomas Koenig, ig25@rz.uni-karlsruhe.de
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Thomas Koenig, ig25@rz.uni-karlsruhe.de
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