2011.03.28 21:34

BFQ-v2 IO Scheduler

오늘은, BFQ-v2에 대해 언급합니다.
BFQ-v2는 BFQ보다 fater~합니다.


BFQ-v2 provides the following four new features. According to my tests on 2.6.37 (extended and run with the valuable help of Mauro Andreolini), none of these features is paid with a loss of aggregate throughput with respect to BFQ-v1.
BFQ-v2 provides the same low-latency guarantees with and without NCQ. In other terms, differently from CFQ, latency guarantees are preserved also in presence of NCQ. For example: under a heavy workload and with BFQ-v2, it takes about two seconds to start konsole with and without NCQ on a medium-rate disk, whereas, under CFQ, it takes about 21 seconds without NCQ and about 2940 seconds (49 minutes) if NCQ is enabled.

The latency for interactive applications is about halved with respect to BFQ-v1.

When the low_latency tunable is set, also soft real-time applications now enjoy reduced latency (audio and video players/servers, music, ...). More in detail, when low-latency was enabled with BFQ-v1, only interactive applications were privileged, at the expense of non-interactive ones. This caused the latency experienced by soft real-time applications to grow, and become comparable to the one guaranteed by CFQ. Now, also soft real-time applications are properly privileged. For example, under heavy load and while other commands are repeatedly invoked, a video player enjoys about five-time-lower frame-drop rate than with CFQ.

A very little minimum bandwidth is now guaranteed to the Idle IO-scheduling class also when the other classes are backlogged, just to prevent the former from starving. In some situations, this starvation in turn happened to cause critical processes to block waiting for the disk indefinitely, which led to system livelock. For example, if konsole was repeatedly started and closed while two files were being read in parallel, then, after a few invocations, Xorg hung and login attempts from textual consoles blocked indefinitely (2.6.37, Ubuntu 10.10, files--the ones read in parallel--stored on a different partition w.r.t. to konsole and libraries). The actual bug is probably outside the block layer; however, differently from CFQ, with BFQ-v2 the system apparently does not hang anymore in this and other medium/heavy-load situations.
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Trackback 0 Comment 1
  1. burberry usa 2013.04.09 20:01 신고 address edit & del reply

    그들은 가난한 신혼부부였다.



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