On Thu, 15 Feb 2024 17:38:38 -0800 Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote: > > What this description lacks is any description of why anyone cares. > > > > The patch clearly decreases overall throughput (speed-vs-latency is a > > common tradeoff). This, please. > > And the "we don't know how to fix this properly so punt it into a > > kernel thread" approach remains lame. For example, the risk that the > > now-liberated allocator can outpace the async freeing, resulting in > > unlimited object windup. > > > Andrew, > > What you are saying about outpacing asyn free is true for v1 and v2 versions of the patch. > > But in this latest version, if another reclaim comes in before the async free has kicked in, > we would be freeing the whole cache directly, same as original code, without waiting > for the async free. It is different from the first version > where you go into the free one at a time mode while waiting for the async free. > That was also my objection to the first two versions as you could be in this > slow free one at a time mode for a long time. > > So now we should not have unlimited object windup. And we would be doing free > in batch of 64, either still in the direct path or in the async path. > OK, thanks, I didn't read closely enough, > If the next swap fault comes in very fast, before the async > free gets a chance to run. It will directly free all the swap > cache in the swap fault the same way as previously. And might it be a win to cancel the async_work in this case? Again, without a clear description of the userspace-visible effects of this problem I am groping in the dark. My hands blindly landed upon the question: the overall effect here is to leave worst-case latency unaltered, but to decrease average latency. Does this satisfy the yet-to-be-described requirements? Also, the V4 patch's quoted quantitative testing results are pasted from the V2 patch's. V2 was a fundamentally different implementation. I think it is fair to say that V4 is "untested", with regard to satisfying its runtime objectives.
