]> git.baikalelectronics.ru Git - kernel.git/commit
readahead: fix sequential read cache miss detection
authorDamien Ramonda <damien.ramonda@intel.com>
Tue, 12 Nov 2013 23:08:16 +0000 (15:08 -0800)
committerLinus Torvalds <torvalds@linux-foundation.org>
Wed, 13 Nov 2013 03:09:09 +0000 (12:09 +0900)
commit4480ba78484ba7988e5cb69118294e529d78af02
tree35ca58b3831270a3015e64297eb2cbf3332439ca
parent61d6821897c11db0c2e4a9ccb7b8ddfedf0b21f0
readahead: fix sequential read cache miss detection

The kernel's readahead algorithm sometimes interprets random read
accesses as sequential and triggers unnecessary data prefecthing from
storage device (impacting random read average latency).

In order to identify sequential cache read misses, the readahead
algorithm intends to check whether offset - previous offset == 1
(trivial sequential reads) or offset - previous offset == 0 (sequential
reads not aligned on page boundary):

  if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)

The current offset is stored in the "offset" variable of type "pgoff_t"
(unsigned long), while previous offset is stored in "ra->prev_pos" of
type "loff_t" (long long).  Therefore, operands of the if statement are
implicitly converted to type long long.  Consequently, when previous
offset > current offset (which happens on random pattern), the if
condition is true and access is wrongly interpeted as sequential.  An
unnecessary data prefetching is triggered, impacting the average random
read latency.

Storing the previous offset value in a "pgoff_t" variable (unsigned
long) fixes the sequential read detection logic.

Signed-off-by: Damien Ramonda <damien.ramonda@intel.com>
Reviewed-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Pierre Tardy <pierre.tardy@intel.com>
Acked-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
mm/readahead.c