On Mon, Sep 17, 2018 at 01:00:58PM -0700, Yang Shi wrote: > On 9/15/18 3:10 AM, Matthew Wilcox wrote: > > Something I've been wondering about for a while is whether we should "sort" > > the readers together. ie if the acquirers look like this: > > > > A write > > B read > > C read > > D write > > E read > > F read > > G write > > > > then we should grant the lock to A, BCEF, D, G rather than A, BC, D, EF, G. > > I'm not sure how much this can help to the real world workload. > > Typically, there are multi threads to contend for one mmap_sem. So, they are > trying to read/write the same address space. There might be dependency or > synchronization among them. Sorting read together might break the > dependency?
I don't think that's true for the mmap_sem. If one thread is trying to get the sem for read then it's a page fault. Another thread trying to get the sem for write is trying to modify the address space. If an application depends on the ordering of an mmap vs a page fault, it has to have its own synchronisation.
