rhel5: pci_mmcfg_init marking 256MB space uncacheable, leading to a slowdown

Solution Verified - Updated -

Issue

  • The following warning is seen in Red Hat Enterprise Linux 5 at dmesg or /var/log/messages.
Warning: pci_mmcfg_init marking 256MB space uncacheable.
MCFG table requires %dMB uncacheable only. Try booting with acpi_mcfg_max_pci_bus_num=on
  • We have severe performance issues on RHEL5, only "some" things seem to be slow
  • We have a severe and sudden 10x performance degradation when reading a fully cached large file out of pagecache on RHEL5, at certain offsets only. The performance problem also occurs reading a memory buffer at certain offsets.

Environment

  • Red Hat Enterprise Linux 5
    • all versions of Red Hat Enterprise Linux 5 may have the issue (but not all systems)
    • Red Hat Enterprise Linux 5.6 and later print the observed message
  • x86_64 architecture
  • Does not occur on RHEL6 or RHEL7 on same hardware

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