On Thu, Mar 28, 2024 at 2:07 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Thu, Mar 28, 2024 at 01:23:42PM -0700, Yosry Ahmed wrote: > > On Thu, Mar 28, 2024 at 12:31 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > > > On Mon, Mar 25, 2024 at 11:50:14PM +0000, Yosry Ahmed wrote: > > > > The current same-filled pages handling supports pages filled with any > > > > repeated word-sized pattern. However, in practice, most of these should > > > > be zero pages anyway. Other patterns should be nearly as common. > > > > > > > > Drop the support for non-zero same-filled pages, but keep the names of > > > > knobs exposed to userspace as "same_filled", which isn't entirely > > > > inaccurate. > > > > > > > > This yields some nice code simplification and enables a following patch > > > > that eliminates the need to allocate struct zswap_entry for those pages > > > > completely. > > > > > > > > There is also a very small performance improvement observed over 50 runs > > > > of kernel build test (kernbench) comparing the mean build time on a > > > > skylake machine when building the kernel in a cgroup v1 container with a > > > > 3G limit: > > > > > > > > base patched % diff > > > > real 70.167 69.915 -0.359% > > > > user 2953.068 2956.147 +0.104% > > > > sys 2612.811 2594.718 -0.692% > > > > > > > > This probably comes from more optimized operations like memchr_inv() and > > > > clear_highpage(). Note that the percentage of zero-filled pages during > > > > this test was only around 1.5% on average, and was not affected by this > > > > patch. Practical workloads could have a larger proportion of such pages > > > > (e.g. Johannes observed around 10% [1]), so the performance improvement > > > > should be larger. > > > > > > > > [1]https://lore.kernel.org/linux-mm/20240320210716.GH294822@xxxxxxxxxxx/ > > > > > > > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > > > > > > This is an interesting direction to pursue, but I actually thinkg it > > > doesn't go far enough. Either way, I think it needs more data. > > > > > > 1) How frequent are non-zero-same-filled pages? Difficult to > > > generalize, but if you could gather some from your fleet, that > > > would be useful. If you can devise a portable strategy, I'd also be > > > more than happy to gather this on ours (although I think you have > > > more widespread zswap use, whereas we have more disk swap.) > > > > I am trying to collect the data, but there are.. hurdles. It would > > take some time, so I was hoping the data could be collected elsewhere > > if possible. > > > > The idea I had was to hook a BPF program to the entry of > > zswap_fill_page() and create a histogram of the "value" argument. We > > would get more coverage by hooking it to the return of > > zswap_is_page_same_filled() and only updating the histogram if the > > return value is true, as it includes pages in zswap that haven't been > > swapped in. > > > > However, with zswap_is_page_same_filled() the BPF program will run in > > all zswap stores, whereas for zswap_fill_page() it will only run when > > needed. Not sure if this makes a practical difference tbh. > > > > > > > > 2) The fact that we're doing any of this pattern analysis in zswap at > > > all strikes me as a bit misguided. Being efficient about repetitive > > > patterns is squarely in the domain of a compression algorithm. Do > > > we not trust e.g. zstd to handle this properly? > > > > I thought about this briefly, but I didn't follow through. I could try > > to collect some data by swapping out different patterns and observing > > how different compression algorithms react. That would be interesting > > for sure. > > > > > > > > I'm guessing this goes back to inefficient packing from something > > > like zbud, which would waste half a page on one repeating byte. > > > > > > But zsmalloc can do 32 byte objects. It's also a batching slab > > > allocator, where storing a series of small, same-sized objects is > > > quite fast. > > > > > > Add to that the additional branches, the additional kmap, the extra > > > scanning of every single page for patterns - all in the fast path > > > of zswap, when we already know that the vast majority of incoming > > > pages will need to be properly compressed anyway. > > > > > > Maybe it's time to get rid of the special handling entirely? > > > > We would still be wasting some memory (~96 bytes between zswap_entry > > and zsmalloc object), and wasting cycling allocating them. This could > > be made up for by cycles saved by removing the handling. We will be > > saving some branches for sure. I am not worried about kmap as I think > > it's a noop in most cases. > > Yes, true. > > > I am interested to see how much we could save by removing scanning for > > patterns. We may not save much if we abort after reading a few words > > in most cases, but I guess we could also be scanning a considerable > > amount before aborting. On the other hand, we would be reading the > > page contents into cache anyway for compression, so maybe it doesn't > > really matter? > > > > I will try to collect some data about this. I will start by trying to > > find out how the compression algorithms handle same-filled pages. If > > they can compress it efficiently, then I will try to get more data on > > the tradeoff from removing the handling. > > I do wonder if this could be overthinking it, too. > > Double checking the numbers on our fleet, a 96 additional bytes for > each same-filled entry would result in a > > 1) p50 waste of 0.008% of total memory, and a > > 2) p99 waste of 0.06% of total memory. > > And this is without us having even thought about trying to make > zsmalloc more efficient for this particular usecase - which might be > the better point of attack, if we think it's actually worth it. > > So my take is that unless removing it would be outright horrible from > a %sys POV (which seems pretty unlikely), IMO it would be fine to just > delete it entirely with a "not worth the maintenance cost" argument. > > If you turn the argument around, and somebody would submit the code as > it is today, with the numbers being what they are above, I'm not sure > we would even accept it! The context guy is here :) Not arguing for one way or another, but I did find the original patch that introduced same filled page handling: https://github.com/torvalds/linux/commit/a85f878b443f8d2b91ba76f09da21ac0af22e07f https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/T/#u The number looks impressive, and there is some detail about the experiment setup, but I can't seem to find what the allocator + compressor used. Which, as Johannes has pointed out, matters a lot. A good compressor (which should work on arguably the most trivial data pattern there is) + a backend allocator that is capable of handling small objects well could make this case really efficient, without resorting to special handling at the zswap level.
