CS Honors Research: Memory Mapped files in Linux Checkpoint 1

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CS Honors ResearchMemory Mapped files in
LinuxCheckpoint 1

• Kousha Najafi
• Prof Eddie Kohler
• Grad Student Steve VanDeBogart
• 12.7.07

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Research Topic

• Explore Linux memory-mapped-file system call
  performance.
• Find and Resolve inconstancies when compared to
  read system call

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mmap vs read (Forwards old)
Reading 256MB file on system with 128MB memory
128MB ram pread mmap
min 4.81s 8.29s
max 5.34s 19.46s
avg 4.97s 13.68s
stdev 0.17s 4.95s
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mmap vs read (Forwards)
Reading 256MB file on system with 128MB memory
128MB ram pread mmap
min 5.19s 8.64s
max 5.56s 10.31s
avg 5.29s 9.49s
stdev 0.11s .51s
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mmap vs read (Backwards old)
Reading 256MB file on system with 128MB memory
128MB ram pread mmap
min 23.20s 17.20s
max 23.47s 25.84s
avg 23.33s 23.02s
stdev 0.08s 3.32s
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mmap vs read (Backwards)
Reading 256MB file on system with 128MB memory
128MB ram pread mmap
min 23.85s 14.36s
max 24.20s 15.48s
avg 23.97s 14.94s
stdev 0.11s .43s
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Previous Checkpoints

• Figure out what causes the slowdown
• Figure out what causes the inconsistencies
• Come up with theory to fix problem
• Try to code a patch to fix problem

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Checkpoint Progress

• Figure out what causes the slowdown
• Slowdown is probably due to the read-ahead
  mechanism used by mmap.
• Figure out what causes the inconsistencies
• Seems to be caused by a random race condition
  involved with calling the read-ahead functions.

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Checkpoints Revisions

• Figure out read-ahead slowdown
• Figure out race condition that causes timing
  inconsistencies
• Come up with theory to fix problem