| Re: Linux Memory Overcommit in OpenVZ [message #31900 is a reply to message #31148] |
Sat, 12 July 2008 13:59   |
charlesl
Messages: 4
Registered: March 2008
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Junior Member |
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It has been nearly a month, almost 1000 views and not a single reply so I guess this has everyone else stumped too (or its so obvious it didn't deserve an answer  ) ...
I wrote a couple of programs to test and I'll share my results. The two programs were:
- memalloc - Allocates memory in 100MB/second increments
- memfill - Allocates and fills memory at rate 100MB/second
I then ran these in a VE and watched its user_beancounters from the HN, and the HN's /proc/meminfo too. Here are my findings:
memalloc
The privvmpages accounting increases until it hits the privvmpages barrier, at which failcnt increases and the program begins to fail.
But /proc/meminfo does not change - the kernel is not counting any allocations made via malloc.
memfill
The privvmpages accounting and physpages/oomguarpages accounting all increase. Since this is a test system not under load, the privvmpages barrier is reached first, its failcnt increases and the program fails.
In this case, /proc/meminfo does change - the kernel is counting used pages.
Conclusion
Note: naturally, "allocated" memory includes "used" memory. An allocation limit is implicitly a usable limit too. I use _bar for barrier and _cur for current (held) values.
- The VE has a maximum allocation limit of privvmpages_bar.
- The VE has a guaranteed allocation limit of guarvmpages_bar.
- The kernel only cares about used memory - the sum of all VE's physpages_cur/oomguarpages_cur (+ HN overhead).
- VEs can allocate all the memory they like up to privvmpages_bar under situations where the total used memory is low.
- VEs using memory above their guarantees may fail above the guarantees under high overall memory consumption.
My only one remaining question is: clearly VEs cannot use arbitrary amounts of memory under high load conditions, but can VEs still allocate up to their privvmpages_bar even under high memory conditions, since the kernel doesn't care about allocations?
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