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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 6.8. These changes include added or updated
proc entries, sysctl, and sysfs default values, boot parameters, kernel configuration options, or any noticeable behavior changes.
- force_hrtimer_reprogram [KNL]
- Force the reprogramming of expired timers in the
hrtimer_reprogram()function. - softirq_2ms_loop [KNL]
- Set
softirqhandling to 2 ms maximum. The default time is the existing Red Hat Enterprise Linux 6 behaviour. - tpm_suspend_pcr=[HW,TPM]
- Specify that, at suspend time, the
tpmdriver should extend the specified principal components regression (PCR) with zeros as a workaround for some chips which fail to flush the last written PCR on aTPM_SaveStateoperation. This guarantees that all the other PCRs are saved.Format: integer pcr id /proc/fs/fscache/statsTable 2.1. class Ops:
new: ini=N Number of async ops initialised changed: rel=N will be equal to ini=N when idle Table 2.2. new class CacheEv
nsp=N Number of object lookups or creations rejected due to a lack of space stl=N Number of stale objects deleted rtr=N Number of objects retired when relinquished cul=N Number of objects culled /proc/sys/net/core/default_qdisc- The default queuing discipline to use for network devices. This allows overriding the default queue discipline of
pfifo_fastwith an alternative. Since the default queuing discipline is created with no additional parameters, it is best suited to queuing disciplines that work well without configuration, for example, a stochastic fair queue (sfq). Do not use queuing disciplines like Hierarchical Token Bucket or Deficit Round Robin, which require setting up classes and bandwidths.Default: pfifo_fast /sys/kernel/mm/ksm/max_page_sharing- Maximum sharing allowed for each KSM page. This enforces a deduplication limit to avoid the virtual memory
rmaplists to grow too large. The minimum value is 2 as a newly created KSM page will have at least two sharers. Thermapwalk has O(N) complexity whereNis the number ofrmap_items, that is virtual mappings that are sharing the page, which is in turn capped bymax_page_sharing. So this effectively spreads the linear O(N) computational complexity fromrmapwalk context over different KSM pages. Theksmdwalk over thestable_nodechainsis also O(N), but N is the number ofstable_nodedups, not the number ofrmap_items, so it has not a significant impact onksmdperformance. In practice the beststable_nodedupscandidate is kept and found at the head of thedupslist. The higher this value the faster KSM merges the memory, because there will be fewerstable_nodedups queued into thestable_nodechain->hlist to check for pruning. And the higher the deduplication factor is, but the slowest the worst casermapwalk could be for any given KSM page. Slowing down thermapwalk means there will be higher latency for certain virtual memory operations happening during swapping, compaction, NUMA balancing, and page migration, in turn decreasing responsiveness for the caller of those virtual memory operations. The scheduler latency of other tasks not involved with the VM operations doing thermapwalk is not affected by this parameter as thermapwalks are always scheduled friendly themselves. /proc/sys/net/core/default_qdisc- The default queuing discipline to use for network devices. This allows overriding the default queue discipline of
pfifo_fastwith an alternative. Since the default queuing discipline is created with no additional parameters so is best suited to queuing disciplines that work well without configuration, for example, a stochastic fair queue (sfq). Do not use queuing disciplines like Hierarchical Token Bucket or Deficit Round Robin which require setting up classes and bandwidths.Default: pfifo_fast /sys/kernel/mm/ksm/stable_node_chains_prune_millisecs- How frequently to walk the whole list of
stable_node"dups" linked in thestable_nodechainsin order to prune stalestable_node. Smaller milllisecs values will free up the KSM metadata with lower latency, but they will makeksmduse more CPU during the scan. This only applies to thestable_nodechains so it is a noop unless a single KSM page hitsmax_page_sharing. In such a case there are nostable_nodechains. /sys/kernel/mm/ksm/stable_node_chains- Number of stable node chains allocated. this is effectively the number of KSM pages that hit the
max_page_sharinglimit. /sys/kernel/mm/ksm/stable_node_dups- Number of stable node dups queued into the
stable_nodechains.

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