06-11-2020, 08:37 AM
Just to clarify - neither zram nor zswap is something new to me, I've used both for years. I'm also quite aware of non-mainline tech called zcache. The above post was merely expression of how impressed I was at how well something as low-powered as PBP works with zram, considering its benchmark scores are around the same level as Core 2 Duo, and it appears to be actually more responsive under the same conditions, which I guess is due to the fact that individual cores are perhaps less powerful, but there are three times as many, allowing to spread the load better.
As to zswap being better than zram - that is a claim I will contest. From my experience in both personal computing and working in big enterprise IT there are no one-fits-all solutions when you're trying to optimize performance, it all depends on usage patterns.
In my own experience, under comparatively low memory usage - oh yeah, zswap is great. And zram handles that situation just as well. Now, you don't get with zswap the situation where once you run out of RAM to keep all your compressed pages + stuff that cannot be paged out you're either dealing with priority inversion you've mentioned or your system slows down to a crawl while OOM killer tries to free up some memory. But in my experience if you're in that spot where zram still has enough compressed space to keep paging yet zswap already starts to fall back to on-disk swap, which I personally hit rather frequently on systems with <8GiB RAM with my usage patterns, zswap results in much worse responsiveness. I'm hypothesizing that the reason behind that worse performance is due to the fact that it has to decompress all those compressed pages before writing them to disk, causing even higher CPU load in addition to I/O contention with the apps I use. I've tried tweaking zswap in various ways, yet invariably I have been getting better performance with zram than with zswap for my usage patterns. But your mileage may vary.
As to zswap being better than zram - that is a claim I will contest. From my experience in both personal computing and working in big enterprise IT there are no one-fits-all solutions when you're trying to optimize performance, it all depends on usage patterns.
In my own experience, under comparatively low memory usage - oh yeah, zswap is great. And zram handles that situation just as well. Now, you don't get with zswap the situation where once you run out of RAM to keep all your compressed pages + stuff that cannot be paged out you're either dealing with priority inversion you've mentioned or your system slows down to a crawl while OOM killer tries to free up some memory. But in my experience if you're in that spot where zram still has enough compressed space to keep paging yet zswap already starts to fall back to on-disk swap, which I personally hit rather frequently on systems with <8GiB RAM with my usage patterns, zswap results in much worse responsiveness. I'm hypothesizing that the reason behind that worse performance is due to the fact that it has to decompress all those compressed pages before writing them to disk, causing even higher CPU load in addition to I/O contention with the apps I use. I've tried tweaking zswap in various ways, yet invariably I have been getting better performance with zram than with zswap for my usage patterns. But your mileage may vary.
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