What is the formula to calculate "CommitLimit" value on Red Hat Enterprise Linux 5 and 6 ?

Solution Unverified - Updated -

Environment

  • Red Hat Enterprise Linux 5
  • Red Hat Enterprise Linux 6
  • CommitLimit

Issue

  • The kernel documentation says on a system with 1G of physical RAM and 7G of swap with a vm.overcommit_ratio of 30 it would yield a CommitLimit of 7.3 G.
  • Calculating the CommitLimit value using above entities and formula given in kernel documentation gives an incorrect result.

Eg:

CommitLimit = ('vm.overcommit_ratio' * Physical RAM) + Swap
            = ( 30 * 1048576 ) + 7340032
            = 38797312
            = 37 GiB

Resolution

  • The correct formula to calculate "CommitLimit" value is;
CommitLimit = ([total RAM pages] - [total huge TLB pages]) * overcommit_ratio / 100 + [total swap pages]

Eg:

CommitLimit = ([total RAM pages] - [total huge TLB pages]) * overcommit_ratio / 100 + [total swap pages]
            = (( 1048576 - 0 ) * 30) / 100 + 7340032
            =  7654604.8
            =  7.3 GiB

Root Cause

  • The formula to calculate "CommitLimit" value mentioned in kernel documentation is incorrect.
CommitLimit = ('vm.overcommit_ratio' * Physical RAM) + Swap

  • RHEL-6: [Private] Bug 1050174 - The formula to calculate "CommitLimit" value mentioned in kernel documentation is incorrect.

  • RHEL-5: [Private] Bug 1050185 - The formula to calculate "CommitLimit" value mentioned in kernel documentation is incorrect

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