Chapter 2. Important Changes to External Kernel Parameters This chapter provides system administrators with a summary of significant changes in the kernel shipped with Red Hat Enterprise Linux 6.7. These changes include added or updated procfs entries, sysfs default values, boot parameters, kernel configuration options, or any noticeable behavior changes. efi_smbios_addr [X86,EFI] Parameter used to specify location of SMBIOS for EFI systems. Used by kexec-tools for kdump . initcall_blacklist [KNL] A comma-separated list of initcall functions that should not be executed. Useful for debugging built-in modules and initcalls. panic_on_warn When enabled (set to 1 ), panic() is called after printing out the WARN() location. This is useful when you want to cause kdump on WARN(). /proc/<pid>/numa_maps Shows memory location, binding policy, and mapping details of each mapping. Mapping details now include the page size in kilobytes ( kernelpagesize_kB ), in addition to mapping type, page usage counters, and node-based page counters. /proc/<pid>/smaps Shows memory consumption for each mapping of a process. The output now includes the kernel flags associated with the particular area of virtual memory ( VmFlags ). Kernel flags are shown as a space-separated list of two-letter codes: Table 2.1. VmFlags Codes Code Description rd readable wr writeable ex executable sh shared mr may read mw may write me may execute ms may share gd stack segment growns down pf pure PFN range dw disabled write to the mapped file lo pages are locked in memory io memory mapped I/O area sr sequential read advise provided rr random read advise provided dc do not copy area on fork de do not expand area on remapping ac area is accountable nr swap space is not reserved for the area ht area uses huge tlb pages nl non-linear mapping ar architecture specific flag dd do not include area into core dump mm mixed map area hg huge page advise flag nh no-huge page advise flag mg mergable advise flag net.ip.ip_no_pmtu_disc Disables Path MTU Discovery. This parameter was previously Boolean; it now takes an Integer as a value. Possible values include: 0 (default) Enables Path MTU Discovery. 1 Disables Path MTU Discovery by setting the PMTU to this destination to the value of min_pmtu when a fragmentation-requiring ICMP is received. To avoid locally-generated fragments, manually increase the value of min_pmtu to the interface on your system that has the smallest MTU. 2 Disables Path MTU Discovery by discarding Path MTU discovery messages. Outgoing frames are handled in the same way as in mode 1 , implicitly setting IP_PMTUDISC_DONT on every created socket. net.ip.ip_forward_use_pmtu Disabled (set to 0 ) by default. When enabled, allows Path MTU Discovery while forwarding. Only enable this if you have user space software that depends on the kernel honoring Path MTU Discovery information. net.core.netdev_rss_key Contains a randomly generated host key used by drivers that enable RSS (Receive Side Scaling). Most drivers use 40 byte keys; this parameter allows keys up to 52 bytes. If no driver has ever called netdev_rss_key_fill(), this file contains null bytes. vm.admin_reserve_kbytes Defines the amount of free memory that should be reserved for users with the cap_sys_admin capability. On x86_64, the default value is 8 MB. This is sufficient when using the default overcommit mode. However, on systems where overcommit is set to never , this should be increased to account for the full size of recovery programs in virtual memory. On x86_64, the minimum useful reserve is about 128 MB. Changes to this parameter take effect whenever an application requests memory.

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