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Comment by brianwawok | Hacker News Reader

For me swap is an error. I would rather a server crash then go into swap and run slow.

If your servers are at 75% CPU and start swapping you are screwed. Time to start adding servers. In this day and age you are wasting a lot of money to be sitting around waiting for swap.

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zswap makes swap useful on the server as it results in gradual slowdown when the system has used all the ram. And if the system runs at 75% of CPU, then there is room for such slowdown.

I hadn't heard of zswap [0]:

"Zswap is a lightweight compressed cache for swap pages. It takes pages that are in the process of being swapped out and attempts to compress them into a dynamically allocated RAM-based memory pool. [...] Zswap is a new feature as of v3.11 [...] zswap is a work in progress and should be considered experimental."

The return of RAM Doubler!

The naming is initially confusing if you're already familiar with ZFS, especially:

"Zswap makes use of zpool for the managing the compressed memory pool"

[0] https://www.kernel.org/doc/Documentation/vm/zswap.txt

It's actually quite useful on many consumer devices, especially those with multiple cores. Most of the time I'm happy I can trade a few CPU cycles for better I/O performance.

yes, we run ZRAM of every single server we have, its fantastic and we don't have swap anywhere. It's worth noting though that vm.swappiness is set to 0 then ZRAM wont be used, it must be set to 1 or higher unless you're using a really old kernel (pre 3.5).

zram[0] is not the same thing as zswap[1].

0. https://en.wikipedia.org/wiki/Zram

1. https://en.wikipedia.org/wiki/Zswap

I have found that zswap behaves much better than zram when the size of the used ram approaches 150% of RAM. In theory zram compresses better as zswap has very simple mapping for the location of its cache of compressed pages leading to space underutilization. But, as it has real backing store, it can evict unused pages allowing bigger chunk of uncompressed memory for running applications.

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