On 12 May 2012 22:04, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> Nick proposed years ago that tmpfs should avoid clearing its pages where
> write will overwrite them with new data, as ramfs has long done. But I
> messed it up and just got bad data. Tried again recently, it works fine.
>
> Here's time output for writing 4GiB 16 times on this Core i5 laptop:
>
> before: real 0m21.169s user 0m0.028s sys 0m21.057s
> real 0m21.382s user 0m0.016s sys 0m21.289s
> real 0m21.311s user 0m0.020s sys 0m21.217s
>
> after: real 0m18.273s user 0m0.032s sys 0m18.165s
> real 0m18.354s user 0m0.020s sys 0m18.265s
> real 0m18.440s user 0m0.032s sys 0m18.337s
>
> ramfs: real 0m16.860s user 0m0.028s sys 0m16.765s
> real 0m17.382s user 0m0.040s sys 0m17.273s
> real 0m17.133s user 0m0.044s sys 0m17.021s
Cool, thanks Hugh! Very big speedup.
>
> Yes, I have done perf reports, but they need more explanation than they
> deserve: in summary, clear_page vanishes, its cache loading shifts into
> copy_user_generic_unrolled; shmem_getpage_gfp goes down, and surprisingly
> mark_page_accessed goes way up - I think because they are respectively
> where the cache gets to be reloaded after being purged by clear or copy.
>
> Suggested-by: Nick Piggin <npiggin@xxxxxxxxx>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
> mm/shmem.c | 20 +++++++++++++++++---
> 1 file changed, 17 insertions(+), 3 deletions(-)
>
> --- 3045N.orig/mm/shmem.c 2012-05-05 10:46:05.732062006 -0700
> +++ 3045N/mm/shmem.c 2012-05-05 10:46:12.316062172 -0700
> @@ -1095,9 +1095,14 @@ repeat:
> shmem_recalc_inode(inode);
> spin_unlock(&info->lock);
>
> - clear_highpage(page);
> - flush_dcache_page(page);
> - SetPageUptodate(page);
> + /*
> + * Let SGP_WRITE caller clear ends if write does not fill page
> + */
> + if (sgp != SGP_WRITE) {
> + clear_highpage(page);
> + flush_dcache_page(page);
> + SetPageUptodate(page);
> + }
> if (sgp == SGP_DIRTY)
> set_page_dirty(page);
> }
> @@ -1307,6 +1312,14 @@ shmem_write_end(struct file *file, struc
> if (pos + copied > inode->i_size)
> i_size_write(inode, pos + copied);
>
> + if (!PageUptodate(page)) {
> + if (copied < PAGE_CACHE_SIZE) {
> + unsigned from = pos & (PAGE_CACHE_SIZE - 1);
> + zero_user_segments(page, 0, from,
> + from + copied, PAGE_CACHE_SIZE);
> + }
> + SetPageUptodate(page);
> + }
> set_page_dirty(page);
> unlock_page(page);
> page_cache_release(page);
> @@ -1768,6 +1781,7 @@ static int shmem_symlink(struct inode *d
> kaddr = kmap_atomic(page);
> memcpy(kaddr, symname, len);
> kunmap_atomic(kaddr);
> + SetPageUptodate(page);
> set_page_dirty(page);
> unlock_page(page);
> page_cache_release(page);
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