On Thu, Feb 29, 2024 at 10:21:26AM +0100, David Hildenbrand wrote: > On 28.02.24 23:56, Khalid Aziz wrote: > > Threads of a process share address space and page tables that allows for > > two key advantages: > > > > 1. Amount of memory required for PTEs to map physical pages stays low > > even when large number of threads share the same pages since PTEs are > > shared across threads. > > > > 2. Page protection attributes are shared across threads and a change > > of attributes applies immediately to every thread without any overhead > > of coordinating protection bit changes across threads. > > > > These advantages no longer apply when unrelated processes share pages. > > Large database applications can easily comprise of 1000s of processes > > that share 100s of GB of pages. In cases like this, amount of memory > > consumed by page tables can exceed the size of actual shared data. > > On a database server with 300GB SGA, a system crash was seen with > > out-of-memory condition when 1500+ clients tried to share this SGA even > > though the system had 512GB of memory. On this server, in the worst case > > scenario of all 1500 processes mapping every page from SGA would have > > required 878GB+ for just the PTEs. > > > > I have sent proposals and patches to solve this problem by adding a > > mechanism to the kernel for processes to use to opt into sharing > > page tables with other processes. We have had discussions on original > > proposal and subsequent refinements but we have not converged on a > > solution. As systems with multi-TB memory and in-memory databases > > are becoming more and more common, this is becoming a significant issue. > > An interactive discussion can help us reach a consensus on how to > > solve this. > > Hi, > > I was hoping for a follow-up to my previous comments from ~4 months ago [1], > so one problem of "not converging" might be "no follow-up discussion". > > Ideally, this session would not focus on mshare as previously discussed at > LSF/MM, but take a step back and discuss requirements and possible > adjustments to the original concept to get something possibly cleaner. I think the concept is clean. Your concept doesn't fit our use case! So essentially what you're asking for is for us to do a lot of work which doesn't solve our problem. You can imagine our lack of enthusiasm for this.
