feat: bypass disk wirte cache when cache-size-mb is zero

[tearfur]

Idea and rationale from #5667 (comment).

[Pentaphon]

Have you done any performance tests with write cache disabled? Any benefit to disabing it or simply making sure that "0" actually disables cache as we would expect?

[tearfur]

My theory is this is probably only helpful to those whose filesystem has its own cache layer to group up transactions. Will have to wait for @blind-oracle's results since he has a filesystem with a cache layer.

Disabling the cache was trivial enough that I am confident it works. But I am not able to test it right now.

[blind-oracle]

I did the test - it seems to help, the speed was closer to 1Gbit/s most of time, but still I can't really go over that much (some 1.1Gbit/s max). The daemon hits 100% and there it stays.

Here's the graph, though it was taken with release flags and might be not very usable. But anyway it seems that the cache is now mostly no-op which is good.

[tearfur]

@blind-oracle Nice. 20-30% improvement is a big improvement. But we better compare Debug build with Debug build to be fair.

When you say your CPU is at 100%, do you mean all cores, or just 1?

Looking at your other flame graphs, the other big part is the arc4 cipher. Can you generate another flame graph with encryption disabled? I want to identify the remaining parts that are eating up resources.

[blind-oracle]

@tearfur

Sure, I can do debug, but it's very slow there. I'll try.

When I say 100% I mean one core. I don't think that transmission can effectively use several cores... at least I never saw >100% cpu usage.

About encryption - I don't think you can disable it, at least the lowest level for it is Tolarable. Hard to say what this means, probably it won't initiate encrypted sessions, but would accept ones?

[tearfur]

When I say 100% I mean one core. I don't think that transmission can effectively use several cores... at least I never saw >100% cpu usage.

Yeah Transmission is mostly single threaded.

About encryption - I don't think you can disable it, at least the lowest level for it is Tolarable. Hard to say what this means, probably it won't initiate encrypted sessions, but would accept ones?

Sure, just set it to Tolerable to get rid of as much arc4 CPU time as possible.

[blind-oracle]

Yeah Transmission is mostly single threaded.

Which is sad, a thread-per-torrent would be quite efficient I guess, they don't really need to share any state.

Here's the graph for debug build & tolerated encryption.
For debug build disabling the cache is visible most significantly - speed went up from 4-5MB/s to 20-30MB/s.

ARC4 is still there, but only around 5% of samples.

[ckerr]

commented @ #5667 (comment)

[ckerr]

Just as a side thought, it's very cool that hotspot exports flame graphs svgs that include mouseover text for each slice. That is super useful

[blind-oracle]

@ckerr these particular graphs were made by flamegraph-rs since hotspot is Linux-only GUI and I don't really have any Linux desktop around...

[ckerr]

@blind-oracle TIL about flamegraph-rs, thanks for the info! That is really handy

[tearfur]

Just as a side thought, it's very cool that hotspot exports flame graphs svgs that include mouseover text for each slice. That is super useful

I agree, they have been very useful. 😁

[ckerr]

Notes: Setting "cache-size-mb": 0 in settings.json now disables the disk write cache.