[RFC] Use zram-swap to save memory on 32 MiB devices

[CodeFetch]

Has anyone tested it, yet? This might mitigate #1243 further.

[ghost]

At some places zram is not suggested e.g. on devices with 4 MB flash, only.

Do not use zram-swap for 4MB flash devices as it increases the amount of firmware space used. It is listed here as it is helpful on machines with very little RAM memory. 

https://openwrt.org/docs/guide-user/additional-software/saving_space#excluding_packages

But i didn't test zram on Openwrt based devices, yet.

[txt-file]

According to https://openwrt.org/packages/table/start?dataflt%5BName_pkg-dependencies*%7E%5D=zram we talk about 15 KB of flash space.

[neocturne]

Including missing dependencies (kmod-lib-lz4, swap-utils, block-mount, libssp, possibly more I overlooked), I count about 90KB for the default zram-swap solution. It might be possible to work with less, but I haven't looked into it in detail.

[CodeFetch]

Unfortunately I don't have any 8/32 MiB device deployed or I would test it.

@skorpy2009 As you're unable to phrase your opinion, I assume you're just a negative person.
While I do bootloader development and write an article for upgrading 4/32 MiB devices to 16/64 MiB for the long term, you're just trolling. I've already upgraded more than 50 WR841Ns and they work better than ever without memory pressure. I'm working on Layer 2 WireGuard which gives 40 MBit/s throughput with these devices. So if you are thinking of trashing your routers, give them to me, please and you're doing good for Freifunk and the environment.

[christf]

I have deployed zram-swap on a few devices as well and did see positive effects on runtime behavior.

Given it needs quite a bit of space - I am not sure it should be on by default. How do you feel about building a package such that there is a choice to include it or not?

[CodeFetch]

@christf Can't we just add packages zram-swap to the device definition of 8/32 MiB devices?

[Adorfer]

@CodeFetch Can this be done via site.mk for ar7xx-tiny-Target?

[CodeFetch]

@Adorfer Yes, I think so, but I haven't tested it:

ifeq ($(GLUON_TARGET),ar71xx-tiny)
GLUON_SITE_PACKAGES += zram-swap
endif

But I'm not sure whether it makes sense for the tiny target as flash memory is sparse. I think it makes more sense for 8/32 MiB devices... Actually I really dislike the idea of compressing RAM, but as a last resort it might be reasonable.

[kevin-olbrich]

I am currently testing this package. I would like to use the existing respondd / yanic / influxdb / grafana setup to get reliable data.
I am building this for all targets, and plan to roll out to 150 devices soon.
Can someone share his grafana RAM usage dashboard?
Nvm: Have been able to create it myself.

[kevin-olbrich]

I've applied the update to an Ubiquiti Nanostation M XW (Update at 2pm):

root@dc-687251721d0d:~# free -h
             total       used       free     shared    buffers     cached
Mem:         59648      30996      28652        232       2316       8628
-/+ buffers/cache:      20052      39596
Swap:        29692          0      29692

root@dc-687251721d0d:~# dmesg | grep zram
[    8.288819] zram: Added device: zram0
[   15.878349] zram0: detected capacity change from 0 to 30408704
[   15.927335] Adding 29692k swap on /dev/zram0.  Priority:-1 extents:1 across:29692k SS

Load has increased which might be caused by compression.

I will test another device with less RAM.

[Adorfer]

from these metrics i understand that the zram had the opposite of the expected effect on 64MB devices?

the following question(s) could be: even if metrics are not looking good:

• Does it perhaps reduce high load (>1) scenarios?
• does it prevent/reduce reboots under memmory preassure on "some nodes"

[kevin-olbrich]

TP-Link TL-WR841N/ND v8 (update ~6pm)

root@dolphin-de01-a0f3c18fc3fc:~# free -h
             total       used       free     shared    buffers     cached
Mem:         27684      24240       3444         76       2076       5424
-/+ buffers/cache:      16740      10944
Swap:        13308         52      13256

root@dolphin-de01-a0f3c18fc3fc:~# dmesg | grep zram
[   10.168613] zram: Added device: zram0
[   13.113622] zram0: detected capacity change from 0 to 13631488
[   13.155239] Adding 13308k swap on /dev/zram0.  Priority:-1 extents:1 across:13308k SS

root@dolphin-de01-a0f3c18fc3fc:~# uptime
 19:18:51 up  1:34,  load average: 0.20, 0.41, 0.27

This node has four mesh partners, (88%, 77%, 73% and 2%) and no clients.

@Adorfer
I never had the problem of sudden reboots (most nodes are permanently online after each update - backed by Respondd statistics).

Maybe Zram allocates the space in RAM (like a ballooning device). This would mean, only data that needs to swap will be compressed (which is fine). This would also confirm why 4/32 devices look better.

[kevin-olbrich]

Another Ubiquiti Nanostation M XW with non-zram firmware:

root@dolphin-de01-hw-802aa86eecbf:~# free -h
             total       used       free     shared    buffers     cached
Mem:         59648      26616      33032        104       2272       7256
-/+ buffers/cache:      17088      42560
Swap:            0          0          0

Same total RAM, less used, no swap.

[kevin-olbrich]

TP-Link TL-WR1043N/ND v2

Currently about ~150 devices received the new update including zram (all targets).

[kevin-olbrich]

I've applied the update to an Ubiquiti Nanostation M XW (Update at 2pm):

root@dc-687251721d0d:~# free -h
             total       used       free     shared    buffers     cached
Mem:         59648      30996      28652        232       2316       8628
-/+ buffers/cache:      20052      39596
Swap:        29692          0      29692

root@dc-687251721d0d:~# dmesg | grep zram
[    8.288819] zram: Added device: zram0
[   15.878349] zram0: detected capacity change from 0 to 30408704
[   15.927335] Adding 29692k swap on /dev/zram0.  Priority:-1 extents:1 across:29692k SS

Load has increased which might be caused by compression.

I will test another device with less RAM.

Same device but RAM usage settled down.

[neocturne]

The output of free is more or less meaningless, what you really want is MemAvailable in /proc/meminfo.

[kevin-olbrich]

The output of free is more or less meaningless, what you really want is MemAvailable in /proc/meminfo.

The diagrams should be fine then:
SELECT mean("memory.total") - mean("memory.available") as used FROM "node" WHERE ("nodeid" =~ /$node$/) AND $timeFilter GROUP BY time($interval)

[Adorfer]

how did you apply the zram? Just by site.mk or by patching gluon/target-files?

[kevin-olbrich]

how did you apply the zram? Just by site.mk or by patching gluon/target-files?

I‘ve used the site.mk-packages approach.

[mweinelt]

TP-Link TL-WR842NDv2 (8 MB Flash, 32 MB RAM)
Left without zram, right with zram.
No clients were connected to the device during the testing period.

Let's say this saves us roughly 8% (2.1M) memory.
The average load goes up from 0.2 to 0.3, peak load is multiple times around 1.0 up from 0.5.

Does that sound like it's worth it? What result would we expect from zram-swap to make use of it?

[Adorfer]

evaluating those free/memavail/load-values "as long as it's non critical" is one metric.
accounting for "reduction of reboots and permanent high-load-scenarios" may be another.

i will try it out on some nodes facing regular "reboots after high load" (OOM and whatever is happening).
in other words: even if the base load-peaking increases, if the lockups or lockup-alike-situations are reduced, that would help.

[CodeFetch]

When I've made tests with the SquashFS thrashing situation it was sometimes a question of 0.5 MB.
2.8 MB will prevent this situation in networks that have grown too big, until people have splitted it into different domains.

[Adorfer]

I tested on some devices and most 32B "wifimesh-only-devices in local clouds" look like that:

1st circle: migration from 2016.2 to 2018.2, a
2nd circle: migration from 2018.2 to 2018.2 with zram enabled.

[CodeFetch]

@Adorfer So it's still bad, but better with zram-swap? Can you post a link to the Grafana page? I can't recognize how high the load is as the graph is cut.

[Adorfer]

https://map.eulenfunk.de/stats/d/000000004/node-byid?orgId=1&var-nodeid=60e327c6f834&from=1559832353091&to=1561228144453
basically all nodes from this cloud which are now on "2019062101-exp / gluon-v2018.2.1-26-g3ae816d"
https://map.ffdus.de/#!v:g;n:60e327c6f834

[mweinelt]

Your perspective is somewhat skewed and zram-swap does not look like an improvement on this device at all.

• The CPU graph is visually limited to 1.0, while the load peaks above 8.0
  

• The memory usage has increased after enabling zram-swap
  

• Uptime has gotten worse, down from days to hours
  

[CodeFetch]

@mweinelt I think you interpreted it wrong. Until the 19.06 they were running v2016, on 20.06 19.06 they were updated to v2018.2 which resulted in high load, memory usage and reboots on 21.06 they were updated to v2018.2 with zram-swap, which resulted in lower load, memory usage and no reboots. Thus zram-swap improved the situation, but it is still worse than v2016.

[rotanid]

i agree, the timeframe has to be changed to see the difference (but this comment does NOT comment on the actual impact)

[CodeFetch]

Left half v2018.2 w/o zram-swap Right half with zram-swap

[CodeFetch]

The reason why this should mitigate the load issue is that the load issue is a thrashing issue caused by the flash read and decompression of LZMA blocks in SquashFS on a page fault.

Decompressing RAM is bad (but it's swap and thus infrequently used pages), but reading from flash and decompressing big LZMA blocks is worse. Thus I recommend enabling this package for at least all devices with 32 MiB RAM. As swap is only used if there is a lack of memory or to cache infrequently used pages and because zram-swap is very fast compared to a hard drive I'd even go further and say: It should not hurt to enable it on all devices.

As you can see here:
https://map.eulenfunk.de/stats/d/000000004/node-byid?orgId=1&var-nodeid=60e327c6f834&from=1561139843410&to=1561228144453

the load has decreased from average 1.8 (peak 9.3) without zram-swap to average 0.4 (peak 2.2) with zram-swap enabled.
Furthermore you can see that the load peak of 2.2 of the device with zram-swap is not due to a lack of memory, but because of a high traffic volume.

[CodeFetch]

My interpretaion of this is a buffer is geeting filled because of many packets, the slab or slub cache needs more space and frees pages and in the middle pages needs to be reread which causes high load. I guess it's slab that needs space, because the traffic goes down. Likely many small packets...

Thus there is still a thrashing-problem, but it's better with zram-swap as the router can recover from it.

Edit: Another topic: We should consider decreasing the ag71xx NAPI weight and ring buffer size. This might help in such situations, too. Better drop packets than thrashing as it will trigger fq_codel on client devices which support it and throttle the rate. But what I can see from this: 32 MiB is not enough in the long-run and domains need to get smaller. zram-swap is a quick fix for getting the network in a state again for doing the needed steps.

[kevin-olbrich]

A little bit OT but might be helpful for others who want to test this:
Adding this package to site.mk (which means it is included for all targets) has low to none risk of bricking nodes.
I ran this upgrade last week for a total of 170 nodes, all came back online after the upgrade.
Another rollout with 340 nodes over 8 domains (2017.x -> 2018.2.1 upgrade) also has been flawless.
IMHO this is safe to try in production.

[CodeFetch]

openwrt/openwrt#1515

[Adorfer]

If there would be some flag in targets to enable this by default on all 32MB-RAM devices via site.mk: Would be great
As long as i do not have this selection, i turn it on for all devices for the moment. since it seems to have no visible negative effect on 200+ production nodes. And it definitly helps the routers in dense (multi-link) wifimesh scenarios with several uplinks per local cloud.

[christf]

so we change the default then? It'd have my vote.

[rotanid]

@mweinelt @NeoRaider ?

[oszilloskop]

FYI:
In our community (ffffm) we provided the ar71xx-tiny target with this package since mid July (~250 4/32 nodes, gluon 2018.2.2). So far there have been no abnormalities.

[rotanid]

@mweinelt @NeoRaider ?
@blocktrron @christf @T-X ?

ACK for 4/32, for 8/32 or NACK?

i can live with either decision, tendency to 8/32

[blocktrron]

@rotanid ACK for 8/32, NACK for 4/32, as flash space is precious there (and the target is deprecated anyway)

[Adorfer]

@blocktrron

1. are there any tests showing problem in flashsize related to zram-swap activated in 4/32-dedevices?
   (Since most of /32 devices are with only 4MB of flash, only puttem them to the few 8MB devices would not be a great help)
2. why not give it to /64 devices running dualband (which can run into near-OOM/high load situations)

[CodeFetch]

@Adorfer

1. are there any tests showing problem in flashsize related to zram-swap activated in 4/32-dedevices?

It just means it should not be a default for 4/32 MB devices in Gluon (and I agree with that). People can still select it manually in their firmware builds.

[rotanid]

Ich habe auf Basis dieser Liste
https://openwrt.org/toh/views/toh_gluon_supported?dataflt%5BRAM+MB*%7E%5D=32&dataflt%5BFlash+MB*%7E%5D=8
nun einen PR angelegt:
#1819

[rotanid]

merged #1819 , closing.