On Mon, Jun 21, 2021 at 1:31 AM SeongJae Park <sj38.park@xxxxxxxxx> wrote: > > From: SeongJae Park <sjpark@xxxxxxxxx> > > This commit introduces a reference implementation of the address space > specific low level primitives for the virtual address space, so that > users of DAMON can easily monitor the data accesses on virtual address > spaces of specific processes by simply configuring the implementation to > be used by DAMON. > > The low level primitives for the fundamental access monitoring are > defined in two parts: > > 1. Identification of the monitoring target address range for the address > space. > 2. Access check of specific address range in the target space. > > The reference implementation for the virtual address space does the > works as below. > > PTE Accessed-bit Based Access Check > ----------------------------------- > > The implementation uses PTE Accessed-bit for basic access checks. That > is, it clears the bit for the next sampling target page and checks > whether it is set again after one sampling period. This could disturb > the reclaim logic. DAMON uses ``PG_idle`` and ``PG_young`` page flags > to solve the conflict, as Idle page tracking does. > > VMA-based Target Address Range Construction > ------------------------------------------- > > Only small parts in the super-huge virtual address space of the > processes are mapped to physical memory and accessed. Thus, tracking > the unmapped address regions is just wasteful. However, because DAMON > can deal with some level of noise using the adaptive regions adjustment > mechanism, tracking every mapping is not strictly required but could > even incur a high overhead in some cases. That said, too huge unmapped > areas inside the monitoring target should be removed to not take the > time for the adaptive mechanism. > > For the reason, this implementation converts the complex mappings to > three distinct regions that cover every mapped area of the address > space. Also, the two gaps between the three regions are the two biggest > unmapped areas in the given address space. The two biggest unmapped > areas would be the gap between the heap and the uppermost mmap()-ed > region, and the gap between the lowermost mmap()-ed region and the stack > in most of the cases. Because these gaps are exceptionally huge in > usual address spaces, excluding these will be sufficient to make a > reasonable trade-off. Below shows this in detail:: > > <heap> > <BIG UNMAPPED REGION 1> > <uppermost mmap()-ed region> > (small mmap()-ed regions and munmap()-ed regions) > <lowermost mmap()-ed region> > <BIG UNMAPPED REGION 2> > <stack> > > Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx> > Reviewed-by: Leonard Foerster <foersleo@xxxxxxxxx> > Reviewed-by: Fernand Sieber <sieberf@xxxxxxxxxx> Acked-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
