New Year, New Kernal

Last week a new Linux kernal was released. The major improvements are a boost to the Ext4 filesystem block size from the traditional 4KB to 1MB.

"This adds supports for bigalloc file systems," Linux kernel developer Ted Ts'o wrote in his initial code commit. "It teaches the mount code just enough about bigalloc superblock fields that it will mount the file system without freaking out that the number of blocks per group is too big."

Ext4 was introduced in the kernal in 2008 as a successor to Ext3. Today, it is often used as the default filesystem. The future filesystems beyond Ext4 include Btrfs.

"This change raises the average disk bandwidth utilization on my test volume from 70 percent to 90 percent," Kernel developer Arne Jansen wrote in his code commit. "On another volume, the time for a test run went down from 89s to 43s."

A new feature debuting in this 3.2 kernal is CFS Bandwidth Control capability. Essentially, this would allow a server to assign a maximum CPU bandwidth to a group.

"The bandwidth allowed for a group is specified using a quota and period," the kernel documentation states. "Within each given 'period' (microseconds), a group is allowed to consume only up to 'quota' microseconds of CPU time. When the CPU bandwidth consumption of a group exceeds this limit (for that period), the tasks belonging to its hierarchy will be throttled and are not allowed to run again until the next period."

Also, the 3.2 kernel implements a new IETF draft specification called, Proportional Rate Reduction for TCP (PRR) to the networking interface.

"PRR is an algorithm that determines TCP's sending rate in fast recovery," Kernel developer Nandita Dukkipati wrote in his commit message. "PRR avoids excessive window reductions and aims for the actual congestion window size at the end of recovery to be as close as possible to the window determined by the congestion control algorithm. PRR also improves accuracy of the amount of data sent during loss recovery."


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