A Status Report on Research in Transparent Informed Prefetching (TIP)

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A Status Report on Research in Transparent
Informed Prefetching (TIP)

• Presented by Hsu Hao Chen

Presented by Hsu Hao Chen
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Outline

• Introduction
• I/O performance is lagging
• No current solution fully addresses read latency
• TIP to reduce latency
• Transparent Informed Prefetching (TIP)
• Implementation
• Evaluation
• Conclusions

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Introduction (1/3)
LFS
But, cache effectiveness is declining
LFS log-structured file systems RAIDs cannot
reduce seek and rotational latencies
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Introduction (2/3)

• Disappointing performance over time
• growing file sizes
• they are accessed too infrequently

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Introduction (3/3)

• Prefetching based on application level
  information is a feasible and effective strategy
  for reducing file access read latency
• TIP reduces application execution time in two
  ways
• By exposing concurrency in the I/O workload
• Overlap I/O with computation or user think time
• Optimize I/O accesses over the larger number of
  outstanding requests
• With knowledge of future I/O requests, TIP and
  make informed cache management decisions that
  lower cache miss rates

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Transparent Informed Prefetching(1/4)

• TIP
• Encapsulate programmer knowledge about future I/O
  requests in a hint
• Transfer hint to file system
• File system uses hints to transparently prefetch
  data and manage resources

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Transparent Informed Prefetching(2/4)

• Obtaining Hints
• Early knowledge of serial file access
• Access patterns part of code algorithm
• large matrix supercomputing read by row, read by
  column
• Hints generated by programmer, compiler,
  profiler

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Transparent Informed Prefetching(3/4)

• Hints generated by compiler (too complex)

SpecHint tool is implemented in 16,000 lines of c
C code SpecHint object files, 4000 lines of
assembly code Data from Automatic I/O Hint
Gereration Through Speculative Execution
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Transparent Informed Prefetching(4/4)

• Users not qualified to give advice
• Advice not portable, disclosure is
• Disclosure allows more flexibility
• Disclosure supports global optimizations

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Implementation

• Implementation of two ways
• Application unmodified
• grep command
• Application modified
• make command
• access pattern profiler

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Implementation- Application unmodified
hcsh for hint-csh
No
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Implementation- Application modified
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Evaluation(1/5)

• Application example
• grep foo 58 files, 1MB
• make xcalc compile link X window calculator
• The local disk tests were conducted on a Sun
  Sparcstation 2 running Mach/BSD Unix 4.3.
• The remote tests were run on two Decstation
  5000/200 also running Mach.

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Evaluation(2/5)
make modified
grep unmodified hcsh for hint-csh
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Evaluation(3/5)
Test Results
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Evaluation(4/5)

• Lessons from tests
• Independent prefetch process overhead too high
• Example context switching, processes schedule,
  system call cost
• Single prefetch process gt no deep prefetch
  queues
• Disk write scheduling often very inefficient
• Sol buffered writes

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Evaluation(5/5)

• Lessons from tests
• Coda ioctl allowed too much prefetching
• thread starvation
• need low-priority prefetching
• premature cache flushing (prefetch data problem)
• need to track consumption

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Conclusions

• TIP offers a powerful mechanism for overcoming
  the I/O crisis
• Hints that disclose, not advise, provide the best
  information and are consistent
• Applicable to local disk and network file servers
• Highlights the importance of combining
  prefetching with buffer management