Johannes Weiner <hannes@xxxxxxxxxxx> writes: > On Wed, Apr 19, 2017 at 03:06:24PM +0800, Huang, Ying wrote: >> From: Huang Ying <ying.huang@xxxxxxxxx> >> >> To swap out THP (Transparent Huage Page), before splitting the THP, >> the swap cluster will be allocated and the THP will be added into the >> swap cache. But it is possible that the THP cannot be split, so that >> we must delete the THP from the swap cache and free the swap cluster. >> To avoid that, in this patch, whether the THP can be split is checked >> firstly. The check can only be done racy, but it is good enough for >> most cases. >> >> With the patchset, the swap out throughput improves 3.6% (from about >> 4.16GB/s to about 4.31GB/s) in the vm-scalability swap-w-seq test case >> with 8 processes. The test is done on a Xeon E5 v3 system. The swap >> device used is a RAM simulated PMEM (persistent memory) device. To >> test the sequential swapping out, the test case creates 8 processes, >> which sequentially allocate and write to the anonymous pages until the >> RAM and part of the swap device is used up. >> >> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> >> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx> >> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> [for can_split_huge_page()] > > How often does this actually happen in practice? Because all that this > protects us from is trying to allocate a swap cluster - which with the > si->free_clusters list really isn't all that expensive - and return it > again. Unless this happens all the time in practice, this optimization > seems misplaced. In addition to allocate/free swap cluster, add/delete to/from the swap cache will be called too. > It's especially a little strange because in the other email I asked > about the need for unlikely() annotations, yet this patch is adding > branches and checks for what seems to be an unlikely condition into > the THP hot path. > > I'd suggest you drop both these optimization attempts unless there is > real data proving that they have a measurable impact. To my surprise too, I found this patch has measurable impact in my test. The swap out throughput improves 3.6% in the vm-scalability swap-w-seq test case with 8 processes. Details are in the original patch description. Best Regards, Huang, Ying -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
