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readahead: update comments to reflect bakeoff results
It turns out that readahead() alone is fastest. Signed-off-by: Zygo Blaxell <bees@furryterror.org>
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src/bees.cc
14
src/bees.cc
@ -231,17 +231,23 @@ bees_readahead(int const fd, off_t offset, size_t size)
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Timer readahead_timer;
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BEESNOTE("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
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BEESTOOLONG("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
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#if 1
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// In the kernel, readahead() is identical to posix_fadvise(..., POSIX_FADV_DONTNEED)
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DIE_IF_NON_ZERO(readahead(fd, offset, size));
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#if 0
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#else
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// Make sure this data is in page cache by brute force
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// This isn't necessary and it might even be slower
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// This isn't necessary and it might even be slower,
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// but the btrfs kernel code does readahead with lower ioprio
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// and might discard the readahead request entirely,
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// so it's maybe, *maybe*, worth doing both.
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BEESNOTE("emulating readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
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while (size) {
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// don't care about multithreaded writes to this buffer--it is garbage anyway
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static uint8_t dummy[BEES_READAHEAD_SIZE];
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size_t this_read_size = min(size, sizeof(dummy));
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// Ignore errors and short reads.
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// It turns out our size parameter isn't all that accurate.
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// Ignore errors and short reads. It turns out our size
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// parameter isn't all that accurate, so we can't use
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// the pread_or_die template.
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(void)!pread(fd, dummy, this_read_size, offset);
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BEESCOUNT(readahead_count);
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BEESCOUNTADD(readahead_bytes, this_read_size);
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