Hello Johannes,

Den fredag 4 april 2025 kl. 22:03:21 +02:00, skrev Johannes Weiner <hannes@cmpxchg.org>:

> Hello Vitaly,

> 

> On Fri, Apr 04, 2025 at 09:28:13PM +0200, Vitaly Wool wrote:

>> zblock is a special purpose allocator for storing compressed pages.

>> It stores integer number of compressed objects per its block. These

>> blocks consist of several physical pages (2**n, i. e. 1/2/4/8).

>> 

>> With zblock, it is possible to densely arrange objects of various sizes

>> resulting in low internal fragmentation. Also this allocator tries to

>> fill incomplete blocks instead of adding new ones, in many cases

>> providing a compression ratio substantially higher than z3fold and zbud

>> (though lower than zmalloc's).

> 

> Do you have zswap/zswapped meminfo metrics from these tests?

Yep, and those look somewhat similar:

 - zblock:

Zswap:            234128 kB

Zswapped:         733216 kB

-  zsmalloc:

Zswap:            286080 kB
Zswapped:         774688 kB

<snip>

>> Since zswap is used starting from minute 21, this gives 9% in average in

>> advantage for zblock.

>

> This might be the linker step swapping out source files that are no

> longer needed. Do you have metrics for zswpout and zswpin as well?

I don't think so, the logs show that the compilation is ongoing. Then again, we're just running out of RAM at that point.

Regarding the zswpin/zswpout, zswpin numbers are marginal with the compilation test and zswpout numbers are slightly bigger for zsmalloc (average ~50k vs ~49k).

The numbers for the stress-ng test (stress-ng --vm 4 --vm-bytes 8G --vm-keep --timeout 10m --metrics-brief) are roughly the same:

- zblock: zswpin 9870143 zswpout 10642176

- zsmalloc: zswpin 10642941 zswpout 10432625

<snip>

> My concern with this allocator, and the other alternatives to zsmalloc

> before, is the following:

> 

> You might be faster at allocating objects. But if storage density is

> worse, it means you have to zswap more pages to meet the same incoming

> allocation demand. That means more object allocations and more

> compression, and often a higher rate of refaults and decompressions.

I understand. It looks like the storage density is comparable, and for 16K pages zblock may even be doing a better job, see above.

~Vitaly