| Message ID | 20230205190036.1730134-1-taejoon.song@lge.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/zswap: try to avoid worst-case scenario on same element pages | expand |
On Mon, 6 Feb 2023 04:00:36 +0900 Taejoon Song <taejoon.song@lge.com> wrote: > The worst-case scenario on finding same element pages is that almost all > elements are same at the first glance but only last few elements are > different. > > Since the same element tends to be grouped from the beginning of the > pages, if we check the first element with the last element before > looping through all elements, we might have some chances to quickly > detect non-same element pages. > > 1. Test is done under LG webOS TV (64-bit arch) > 2. Dump the swap-out pages (~819200 pages) > 3. Analyze the pages with simple test script which counts the iteration > number and measures the speed at off-line > > Under 64-bit arch, the worst iteration count is PAGE_SIZE / 8 bytes = > 512. The speed is based on the time to consume page_same_filled() > function only. The result, on average, is listed as below: > > Num of Iter Speed(MB/s) > Looping-Forward (Orig) 38 99265 > Looping-Backward 36 102725 > Last-element-check (This Patch) 33 125072 > > The result shows that the average iteration count decreases by 13% and > the speed increases by 25% with this patch. This patch does not > increase the overall time complexity, though. > > I also ran simpler version which uses backward loop. Just looping > backward also makes some improvement, but less than this patch. Seems a nice change. > This patch was already applied to zram on mainline v5.6-rc1: > https://lkml.org/lkml/2019/12/3/1422 I redid this a little: A similar change has already been made to zram in 90f82cbfe502 ("zram: try to avoid worst-case scenario on same element pages"). > ... > --- a/mm/zswap.c > +++ b/mm/zswap.c > @@ -1073,15 +1073,23 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle) > > static int zswap_is_page_same_filled(void *ptr, unsigned long *value) > { > - unsigned int pos; > unsigned long *page; > + unsigned long val; > + unsigned int pos, last_pos = PAGE_SIZE / sizeof(*page) - 1; > > page = (unsigned long *)ptr; > - for (pos = 1; pos < PAGE_SIZE / sizeof(*page); pos++) { > - if (page[pos] != page[0]) > + val = page[0]; > + > + if (val != page[last_pos]) > + return 0; > + > + for (pos = 1; pos < last_pos; pos++) { > + if (val != page[pos]) > return 0; > } > - *value = page[0]; > + > + *value = val; > + > return 1; > }
diff --git a/mm/zswap.c b/mm/zswap.c index f6c89049cf70..2f0ebd8bc620 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -1073,15 +1073,23 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle) static int zswap_is_page_same_filled(void *ptr, unsigned long *value) { - unsigned int pos; unsigned long *page; + unsigned long val; + unsigned int pos, last_pos = PAGE_SIZE / sizeof(*page) - 1; page = (unsigned long *)ptr; - for (pos = 1; pos < PAGE_SIZE / sizeof(*page); pos++) { - if (page[pos] != page[0]) + val = page[0]; + + if (val != page[last_pos]) + return 0; + + for (pos = 1; pos < last_pos; pos++) { + if (val != page[pos]) return 0; } - *value = page[0]; + + *value = val; + return 1; }
The worst-case scenario on finding same element pages is that almost all elements are same at the first glance but only last few elements are different. Since the same element tends to be grouped from the beginning of the pages, if we check the first element with the last element before looping through all elements, we might have some chances to quickly detect non-same element pages. 1. Test is done under LG webOS TV (64-bit arch) 2. Dump the swap-out pages (~819200 pages) 3. Analyze the pages with simple test script which counts the iteration number and measures the speed at off-line Under 64-bit arch, the worst iteration count is PAGE_SIZE / 8 bytes = 512. The speed is based on the time to consume page_same_filled() function only. The result, on average, is listed as below: Num of Iter Speed(MB/s) Looping-Forward (Orig) 38 99265 Looping-Backward 36 102725 Last-element-check (This Patch) 33 125072 The result shows that the average iteration count decreases by 13% and the speed increases by 25% with this patch. This patch does not increase the overall time complexity, though. I also ran simpler version which uses backward loop. Just looping backward also makes some improvement, but less than this patch. This patch was already applied to zram on mainline v5.6-rc1: https://lkml.org/lkml/2019/12/3/1422 Signed-off-by: Taejoon Song <taejoon.song@lge.com> --- mm/zswap.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-)