[uml-devel] Re: does O_DIRECT hinder greatly file reading performance but not writing??

[<lyo...@cs...>]

Sorry for the repeated posts, but i figured out that my email body was not
posted. My appologies for the spam.

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Hi All,

I am kind of new to UML and I am experiencing a very strange performance
hinder with it. I am running two benchmarks; one that writes a 50Mbyte file
and the other one reads a 50Mbyte file. I tried the benchmarks with and
without setting the O_DIRECT flag, in which cases the whole task is done in a
matter of seconds(10-30 seconds) except when reading the file with O_DIRECT
flag on. In this case, it takes 18-20 min!!! I am running UML 2.4.19 on top of
kernel 2.4.25. I have time output like that (average of 20 runs)


Operation              UserTime(sec)   SysTime(sec)  WallClockTime(sec)

Write Without O_DIRECT    5.612000  0.117500  27.164500

Write With O_DIRECT       10.007000 0.017000 22.199500

Read without O_DIRECT     3.035000 0.091000 7.760000

Read With O_DIRECT        21.492000 29.680000 1133.054000

I would appreciate it very much if somebody would explain to me how to solve
that strange performance problem, or even explain why does it happen for
reading with O_DIRECT but not without !!??

Thanks allot,
Lamia Youseff
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