From owner-freebsd-fs@FreeBSD.ORG  Wed Aug 31 07:38:38 2011
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Date: Wed, 31 Aug 2011 11:38:33 +0400
From: Lev Serebryakov <lev@serebryakov.spb.ru>
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To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us>
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Cc: freebsd-fs@freebsd.org
Subject: Re: Very inconsistent (read) speed on UFS2
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Hello, Bob.
You wrote 31 =E0=E2=E3=F3=F1=F2=E0 2011 =E3., 2:40:07:

>>>   I'll try this experiment with mmap() and touching every 4096-th byte =
of
>>> mapped memory instead of read(2).
>>  Strange enough, it gives only 40-50MiB/s and results are very
>> consistent.
>>
>>  It really surprise me. I didn't think, that there will be so much
>> difference, I was sure, that it will be almost equivalent speed.
> FreeBSD does not seem to default to sequential read-ahead when memory
> mapping is used with sequential page access.  Try using madvise()=20
> with the MADV_SEQUENTIAL option and see if it helps.
  It were results with MADV_SEQUENTIAL. Code looks like this: (error
checking is skipped here, but not in real code, of course):

fd =3D open(fileName, O_RDONLY | O_DIRECT);
buf =3D mmap(NULL, fileSize, PROT_READ, 0, fd, 0);
madvise(buf, fileSize, MADV_SEQUENTIAL);
gettimeofday(&start, NULL);
for (rd =3D 0; rd < fileSize; rd +=3D 4096)
    c =3D buf[rd];
gettimeofday(&end, NULL);
munmap(buf, fileSize);
close(fd);

> There are also MADV_WILLNEED, MADV_DONTNEED, and MADV_FREE.  Careful
> use of these options can help performance quite a lot when data is=20
> large compared to memory.
  It is too complex for simple linear read test :)

--=20
// Black Lion AKA Lev Serebryakov <lev@serebryakov.spb.ru>