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Date: Sun, 30 Oct 2011 15:52:31 +0400
From: Lev Serebryakov <lev@FreeBSD.org>
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To: Adrian Chadd <adrian@freebsd.org>
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Cc: arch@freebsd.org, lev@freebsd.org
Subject: Re: [PATCH] fadvise(2) system call
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Hello, Adrian.
You wrote 30 =EE=EA=F2=FF=E1=F0=FF 2011 =E3., 15:44:43:

>> =A0 I don't know. But I could show your results of typical ktrace, where
>> =A0transmission-daemon spends 590 seconds of 600 (Wall time) in
>> =A0pread() syscall , and real read speed is only about 30Mbit/s (and
>> =A0this FS could be read at speed about 300MiB/s, 100 times faster,
>> =A0with linear read).
> Is the app doing multi-threaded disk IO? How random is the IO?
  No, it is single-threaded, but I/O is very random one. This single
thread serves many (up to 128 in my setup, and this limit often
reached) clients. Typical "logical" block for client is about 1-2MiB,
and many clients download several consequent logical blocks. But basic
I/O size in transmission is only 32Kb.

> How is fadvise being used in the torrent client? Is it being used to
> force in data into cache?
  Yep. With fadvice() transmission notify kernel about logical block
(1-2MiB, depends on torrent itself) as "will need" and read from it
when client ready to receive next 32KiB portion. Without fadvice() it
looks like completely random I/O with 32KiB blocks (with total size of
files about 1TiB in my case!).

  I have very large vfs.read_max, but it doesn't help much :(

--=20
// Black Lion AKA Lev Serebryakov <lev@FreeBSD.org>