On Fri, Aug 04, 2017 at 12:54:41PM +0900, Minchan Kim wrote: > On Thu, Aug 03, 2017 at 03:13:35PM -0600, Ross Zwisler wrote: > > On Thu, Aug 03, 2017 at 09:13:15AM +0900, Minchan Kim wrote: > > > Hi Ross, > > > > > > On Wed, Aug 02, 2017 at 04:13:59PM -0600, Ross Zwisler wrote: > > > > On Fri, Jul 28, 2017 at 10:31:43AM -0700, Matthew Wilcox wrote: > > > > > On Fri, Jul 28, 2017 at 10:56:01AM -0600, Ross Zwisler wrote: > > > > > > Dan Williams and Christoph Hellwig have recently expressed doubt > > > > > > about > > > > > > whether the rw_page() interface made sense for synchronous memory > > > > > > drivers > > > > > > [1][2]. It's unclear whether this interface has any performance > > > > > > benefit > > > > > > for these drivers, but as we continue to fix bugs it is clear that > > > > > > it does > > > > > > have a maintenance burden. This series removes the rw_page() > > > > > > implementations in brd, pmem and btt to relieve this burden. > > > > > > > > > > Why don't you measure whether it has performance benefits? I don't > > > > > understand why zram would see performance benefits and not other > > > > > drivers. > > > > > If it's going to be removed, then the whole interface should be > > > > > removed, > > > > > not just have the implementations removed from some drivers. > > > > > > > > Okay, I've run a bunch of performance tests with the PMEM and with BTT > > > > entry > > > > points for rw_pages() in a swap workload, and in all cases I do see an > > > > improvement over the code when rw_pages() is removed. Here are the > > > > results > > > > from my random lab box: > > > > > > > > Average latency of swap_writepage() > > > > +------+------------+---------+-------------+ > > > > | | no rw_page | rw_page | Improvement | > > > > +-------------------------------------------+ > > > > | PMEM | 5.0 us | 4.7 us | 6% | > > > > +-------------------------------------------+ > > > > | BTT | 6.8 us | 6.1 us | 10% | > > > > +------+------------+---------+-------------+ > > > > > > > > Average latency of swap_readpage() > > > > +------+------------+---------+-------------+ > > > > | | no rw_page | rw_page | Improvement | > > > > +-------------------------------------------+ > > > > | PMEM | 3.3 us | 2.9 us | 12% | > > > > +-------------------------------------------+ > > > > | BTT | 3.7 us | 3.4 us | 8% | > > > > +------+------------+---------+-------------+ > > > > > > > > The workload was pmbench, a memory benchmark, run on a system where I > > > > had > > > > severely restricted the amount of memory in the system with the 'mem' > > > > kernel > > > > command line parameter. The benchmark was set up to test more memory > > > > than I > > > > allowed the OS to have so it spilled over into swap. > > > > > > > > The PMEM or BTT device was set up as my swap device, and during the > > > > test I got > > > > a few hundred thousand samples of each of swap_writepage() and > > > > swap_writepage(). The PMEM/BTT device was just memory reserved with the > > > > memmap kernel command line parameter. > > > > > > > > Thanks, Matthew, for asking for performance data. It looks like > > > > removing this > > > > code would have been a mistake. > > > > > > By suggestion of Christoph Hellwig, I made a quick patch which does IO > > > without > > > dynamic bio allocation for swap IO. Actually, it's not formal patch to be > > > worth to send mainline yet but I believe it's enough to test the > > > improvement. > > > > > > Could you test patchset on pmem and btt without rw_page? > > > > > > For working the patch, block drivers need to declare it's synchronous IO > > > device via BDI_CAP_SYNC but if it's hard, you can just make every swap IO > > > comes from (sis->flags & SWP_SYNC_IO) with removing condition check > > > > > > if (!(sis->flags & SWP_SYNC_IO)) in swap_[read|write]page. > > > > > > Patchset is based on 4.13-rc3. > > > > Thanks for the patch, here are the updated results from my test box: > > > > Average latency of swap_writepage() > > +------+------------+---------+---------+ > > | | no rw_page | minchan | rw_page | > > +---------------------------------------- > > | PMEM | 5.0 us | 4.98 us | 4.7 us | > > +---------------------------------------- > > | BTT | 6.8 us | 6.3 us | 6.1 us | > > +------+------------+---------+---------+ > > > > Average latency of swap_readpage() > > +------+------------+---------+---------+ > > | | no rw_page | minchan | rw_page | > > +---------------------------------------- > > | PMEM | 3.3 us | 3.27 us | 2.9 us | > > +---------------------------------------- > > | BTT | 3.7 us | 3.44 us | 3.4 us | > > +------+------------+---------+---------+ > > > > I've added another digit in precision in some cases to help differentiate > > the > > various results. > > > > In all cases your patches did perform better than with the regularly > > allocated > > BIO, but again for all cases the rw_page() path was the fastest, even if > > only > > marginally. > > Thanks for the testing. Your testing number is within noise level? > > I cannot understand why PMEM doesn't have enough gain while BTT is significant > win(8%). I guess no rw_page with BTT testing had more chances to wait bio > dynamic > allocation and mine and rw_page testing reduced it significantly. However, > in no rw_page with pmem, there wasn't many cases to wait bio allocations due > to the device is so fast so the number comes from purely the number of > instructions has done. At a quick glance of bio init/submit, it's not trivial > so indeed, i understand where the 12% enhancement comes from but I'm not sure > it's really big difference in real practice at the cost of maintaince burden.
I tested pmbench 10 times in my local machine(4 core) with zram-swap. In my machine, even, on-stack bio is faster than rw_page. Unbelievable. I guess it's really hard to get stable result in severe memory pressure. It would be a result within noise level(see below stddev). So, I think it's hard to conclude rw_page is far faster than onstack-bio. rw_page avg 5.54us stddev 8.89% max 6.02us min 4.20us onstack bio avg 5.27us stddev 13.03% max 5.96us min 3.55us
