[Bugme-janitors] [Bug 10756] New: many pre-mature anticipation timeouts in anticipatory I/O scheduler

[bugme-daemon at bugzilla.kernel.org]

http://bugzilla.kernel.org/show_bug.cgi?id=10756

           Summary: many pre-mature anticipation timeouts in anticipatory
                    I/O scheduler
           Product: IO/Storage
           Version: 2.5
     KernelVersion: 2.6.23
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Block Layer
        AssignedTo: axboe at kernel.dk
        ReportedBy: chuanpengli at yahoo.com
                CC: io_other at kernel-bugs.osdl.org


Latest working kernel version: N/A
Earliest failing kernel version: 2.6.13
Distribution: www.kernel.org
Hardware Environment: IBM eServer: dual 2G Xeon processors;IBM 36GB SCSI drive
Software Environment: Redhat 9: gcc 3.2.2 
Problem Description:
  Starting form 2.6.13, the switch of kernel timer frequency HZ from 1000 to
250 
results in "default_antic_expire = 1 tick". 1 tick is 4 ms, BUT the
anticipation timeout can occur anywhere from 0 to 4 ms, because the timer may
be started anytime from 0 to 4 ms before the next system timer interrupt. In
practice, I observe anticipation timeout as short as 100 micro-seconds using
the LTT trace tool. Compared with HZ=1000, the new frequency (HZ=250) causes
frequent pre-mature anticipation timeouts and degraded I/O throughput under
concurrent I/O workload. I suggest to set the "default_antic_expire" to 2 when
its value is calculated as 1. (see source "block/as-iosched.c")
Steps to reproduce: 
  (1) run a concurrent server with I/O-bound workload, such as a
micro-benchmark that sequentially reads 256 KB from random locations in
randomly chosen files. 
  (2) I/O throughput at HZ=250 is 10-15% lower than HZ=1000
  (3) At HZ=250, a lot of anticipation timeouts can be observed using trace
tools such as LTT.


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