Mechanisms

Address Space Specific Low Level Access Monitoring Primitives

The target address space independent core logics of DAMON, which mainly controls the accuracy/overhead of the monitoring, are separated from the address space dependent low level access monitoring primitives. The primitives are defined in two parts as below.

  1. Identification of the monitoring target address range for the address space.
  2. Access check of specific address range in the target space.

For flexible support of various address spaces, DAMON’s API further export an interface for configuration of the primitives. Therefore, anyone can configure DAMON with appropriate implementations for their use cases and even use their own implementation if necessary. In this way, DAMON can be expanded for any address space while keeping the optimized performance. For example, physical memory, virtual memory, swap space, those for specific processes, NUMA nodes, files, and backing devices would be supportable. Also, if some architectures or kernel module support special access check primitives for specific address space, those will be easily configurable.

DAMON currently provides an implementation of the primitives for the virtual address space. It uses VMA for the target address range identification and PTE Accessed bit for the access check.

Below four sections describe the address independent core mechanisms and the five knobs for tuning, that is, sampling interval, aggregation interval, regions update interval, minimum number of regions, and maximum number of regions. After that, more details about the DAMON’s reference implementation of the primitives for the virtual address space follows.

Basic Access Check

The output of DAMON says what pages are how frequently accessed for a given duration. The resolution of the access frequency is controlled by setting sampling interval and aggregation interval. In detail, DAMON checks access to each page per sampling interval and aggregates the results. In other words, counts the number of the accesses to each page. After each aggregation interval passes, DAMON calls callback functions that previously registered by users so that users can read the aggregated results and then clears the results. This can be described in below simple pseudo-code:

while monitoring_on:
    for page in monitoring_target:
        if accessed(page):
            nr_accesses[page] += 1
    if time() % aggregation_interval == 0:
        for callback in user_registered_callbacks:
            callback(monitoring_target, nr_accesses)
        for page in monitoring_target:
            nr_accesses[page] = 0
    sleep(sampling interval)

The monitoring overhead of this mechanism will arbitrarily increase as the size of the target workload grows.

Region Based Sampling

To avoid the unbounded increase of the overhead, DAMON groups adjacent pages that assumed to have the same access frequencies into a region. As long as the assumption (pages in a region have the same access frequencies) is kept, only one page in the region is required to be checked. Thus, for each sampling interval, DAMON randomly picks one page in each region, waits for one sampling interval, checks whether the page is accessed meanwhile, and increases the access frequency of the region if so. Therefore, the monitoring overhead is controllable by setting the number of regions. DAMON allows users to set the minimum and the maximum number of regions for the trade-off.

This scheme, however, cannot preserve the quality of the output if the assumption is not guaranteed.

Adaptive Regions Adjustment

At the beginning of the monitoring, DAMON constructs the initial regions by evenly splitting the monitoring target memory region into the user-specified minimum number of regions. In this initial state, the assumption is normally not kept and therefore the quality would be low. To keep the assumption as much as possible, DAMON adaptively merges and splits each region based on their access frequency.

For each aggregation interval, it compares the access frequencies of adjacent regions and merges those if the frequency difference is small. Then, after it reports and clears the aggregated access frequency of each region, it splits each region into two or three regions if the total number of regions will not exceed the user-specified maximum number of regions after the split.

In this way, DAMON provides its best-effort quality and minimal overhead while keeping the bounds users set for their trade-off.

Handling Dynamic Target Space Changes

The monitoring target address range could dynamically changed. For example, virtual memory could be dynamically mapped and unmapped. Physical memory could be hot-plugged.

As the changes could be quite frequent in some cases, DAMON checks the dynamic memory mapping changes and applies it to the abstracted target area only for each of a user-specified time interval (regions update interval).

Virtual Address Space Specific Low Primitives

This is for the DAMON’s reference implementation of the virtual memory address specific low level primitive only.

PTE Accessed-bit Based Access Check

The implementation uses PTE Accessed-bit for basic access checks. That is, it clears the bit for next sampling target page and checks whether it set again after one sampling period. To avoid disturbing other Accessed bit users such as the reclamation logic, this implementation adjusts the PG_Idle and PG_Young appropriately, as same to the ‘Idle Page Tracking’.

VMA-based Target Address Range Construction

Only small parts in the super-huge virtual address space of the processes are mapped to the 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. 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>