On Tue, Aug 10, 2021 at 08:51:01AM -0700, Dave Hansen wrote: > In other words, I buy the boot speed argument. But, I don't buy the > "this saves memory long term" argument at all. Okay, that's a fair enough. I guess there's *some* workloads that may have memory footprint reduced, but I agree it's minority. > >> I had expected this series, but I also expected it to be connected to > >> CONFIG_DEFERRED_STRUCT_PAGE_INIT somehow. Could you explain a bit how > >> this problem is different and demands a totally orthogonal solution? > >> > >> For instance, what prevents us from declaring: "Memory is accepted at > >> the time that its 'struct page' is initialized" ? Then, we use all the > >> infrastructure we already have for DEFERRED_STRUCT_PAGE_INIT. > > > > That was my first thought too and I tried it just to realize that it is > > not what we want. If we would accept page on page struct init it means we > > would make host allocate all memory assigned to the guest on boot even if > > guest actually use small portion of it. > > > > Also deferred page init only allows to scale memory accept across multiple > > CPUs, but doesn't allow to get to userspace before we done with it. See > > wait_for_completion(&pgdat_init_all_done_comp). > > That's good information. It's a refinement of the "I want to boot > faster" requirement. What you want is not just going _faster_, but > being able to run userspace before full acceptance has completed. > > Would you be able to quantify how fast TDX page acceptance is? Are we > talking about MB/s, GB/s, TB/s? This series is rather bereft of numbers > for a feature which making a performance claim. > > Let's say we have a 128GB VM. How much does faster does this approach > reach userspace than if all memory was accepted up front? How much > memory _could_ have been accepted at the point userspace starts running? Acceptance code is not optimized yet: we accept memory in 4k chunk which is very slow because hypercall overhead dominates the picture. As of now, kernel boot time of 1 VCPU and 64TiB VM with upfront memory accept is >20 times slower than with this lazy memory accept approach. The difference is going to be substantially lower once we get it optimized properly. -- Kirill A. Shutemov
