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readahead: update comments to reflect bakeoff results

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It turns out that readahead() alone is fastest.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2021-11-16 01:20:16 -05:00
parent 6325f9ed72
commit 5e379b4c48
1 changed files with 10 additions and 4 deletions
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14
src/bees.cc
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@ -231,17 +231,23 @@ bees_readahead(int const fd, off_t offset, size_t size)
Timer readahead_timer; Timer readahead_timer;
BEESNOTE("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size)); BEESNOTE("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
BEESTOOLONG("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size)); BEESTOOLONG("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
#if 1
// In the kernel, readahead() is identical to posix_fadvise(..., POSIX_FADV_DONTNEED) // In the kernel, readahead() is identical to posix_fadvise(..., POSIX_FADV_DONTNEED)
DIE_IF_NON_ZERO(readahead(fd, offset, size)); DIE_IF_NON_ZERO(readahead(fd, offset, size));
#if 0 #else
// Make sure this data is in page cache by brute force // Make sure this data is in page cache by brute force
// This isn't necessary and it might even be slower // This isn't necessary and it might even be slower,
// but the btrfs kernel code does readahead with lower ioprio
// and might discard the readahead request entirely,
// so it's maybe, *maybe*, worth doing both.
BEESNOTE("emulating readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size)); BEESNOTE("emulating readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
while (size) { while (size) {
// don't care about multithreaded writes to this buffer--it is garbage anyway
static uint8_t dummy[BEES_READAHEAD_SIZE]; static uint8_t dummy[BEES_READAHEAD_SIZE];
size_t this_read_size = min(size, sizeof(dummy)); size_t this_read_size = min(size, sizeof(dummy));
// Ignore errors and short reads. // Ignore errors and short reads. It turns out our size
// It turns out our size parameter isn't all that accurate. // parameter isn't all that accurate, so we can't use
// the pread_or_die template.
(void)!pread(fd, dummy, this_read_size, offset); (void)!pread(fd, dummy, this_read_size, offset);
BEESCOUNT(readahead_count); BEESCOUNT(readahead_count);
BEESCOUNTADD(readahead_bytes, this_read_size); BEESCOUNTADD(readahead_bytes, this_read_size);