LKP reports that v4.2 commit afa2db2fb6f1 ("tmpfs: truncate prealloc blocks past i_size") causes a 14.5% slowdown in the AIM9 creat-clo benchmark.
creat-clo does just what you'd expect from the name, and creat's O_TRUNC on 0-length file does indeed get into more overhead now shmem_setattr() tests "0 <= 0" instead of "0 < 0". I'm not sure how much we care, but I think it would not be too VW-like to add in a check for whether any pages (or swap) are allocated: if none are allocated, there's none to remove from the radix_tree. At first I thought that check would be good enough for the unmaps too, but no: we should not skip the unlikely case of unmapping pages beyond the new EOF, which were COWed from holes which have now been reclaimed, leaving none. This gives me an 8.5% speedup: on Haswell instead of LKP's Westmere, and running a debug config before and after: I hope those account for the lesser speedup. And probably someone has a benchmark where a thousand threads keep on stat'ing the same file repeatedly: forestall that report by adjusting v4.3 commit 44a30220bc0a ("shmem: recalculate file inode when fstat") not to take the spinlock in shmem_getattr() when there's no work to do. Reported-by: Ying Huang <ying.hu...@linux.intel.com> Signed-off-by: Hugh Dickins <hu...@google.com> --- mm/shmem.c | 22 ++++++++++++++-------- 1 file changed, 14 insertions(+), 8 deletions(-) --- 4.3-rc7/mm/shmem.c 2015-09-12 18:30:20.857039763 -0700 +++ linux/mm/shmem.c 2015-10-25 11:49:19.931973850 -0700 @@ -548,12 +548,12 @@ static int shmem_getattr(struct vfsmount struct inode *inode = dentry->d_inode; struct shmem_inode_info *info = SHMEM_I(inode); - spin_lock(&info->lock); - shmem_recalc_inode(inode); - spin_unlock(&info->lock); - + if (info->alloced - info->swapped != inode->i_mapping->nrpages) { + spin_lock(&info->lock); + shmem_recalc_inode(inode); + spin_unlock(&info->lock); + } generic_fillattr(inode, stat); - return 0; } @@ -586,10 +586,16 @@ static int shmem_setattr(struct dentry * } if (newsize <= oldsize) { loff_t holebegin = round_up(newsize, PAGE_SIZE); - unmap_mapping_range(inode->i_mapping, holebegin, 0, 1); - shmem_truncate_range(inode, newsize, (loff_t)-1); + if (oldsize > holebegin) + unmap_mapping_range(inode->i_mapping, + holebegin, 0, 1); + if (info->alloced) + shmem_truncate_range(inode, + newsize, (loff_t)-1); /* unmap again to remove racily COWed private pages */ - unmap_mapping_range(inode->i_mapping, holebegin, 0, 1); + if (oldsize > holebegin) + unmap_mapping_range(inode->i_mapping, + holebegin, 0, 1); } } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/