1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ========================== 3 ==========================
4 Frequently Asked Questions 4 Frequently Asked Questions
5 ========================== 5 ==========================
6 6
>> 7 Why a new subsystem, instead of extending perf or other user space tools?
>> 8 =========================================================================
>> 9
>> 10 First, because it needs to be lightweight as much as possible so that it can be
>> 11 used online, any unnecessary overhead such as kernel - user space context
>> 12 switching cost should be avoided. Second, DAMON aims to be used by other
>> 13 programs including the kernel. Therefore, having a dependency on specific
>> 14 tools like perf is not desirable. These are the two biggest reasons why DAMON
>> 15 is implemented in the kernel space.
>> 16
>> 17
>> 18 Can 'idle pages tracking' or 'perf mem' substitute DAMON?
>> 19 =========================================================
>> 20
>> 21 Idle page tracking is a low level primitive for access check of the physical
>> 22 address space. 'perf mem' is similar, though it can use sampling to minimize
>> 23 the overhead. On the other hand, DAMON is a higher-level framework for the
>> 24 monitoring of various address spaces. It is focused on memory management
>> 25 optimization and provides sophisticated accuracy/overhead handling mechanisms.
>> 26 Therefore, 'idle pages tracking' and 'perf mem' could provide a subset of
>> 27 DAMON's output, but cannot substitute DAMON.
>> 28
>> 29
7 Does DAMON support virtual memory only? 30 Does DAMON support virtual memory only?
8 ======================================= 31 =======================================
9 32
10 No. The core of the DAMON is address space in 33 No. The core of the DAMON is address space independent. The address space
11 specific monitoring operations including monit 34 specific monitoring operations including monitoring target regions
12 constructions and actual access checks can be 35 constructions and actual access checks can be implemented and configured on the
13 DAMON core by the users. In this way, DAMON u 36 DAMON core by the users. In this way, DAMON users can monitor any address
14 space with any access check technique. 37 space with any access check technique.
15 38
16 Nonetheless, DAMON provides vma/rmap tracking 39 Nonetheless, DAMON provides vma/rmap tracking and PTE Accessed bit check based
17 implementations of the address space dependent 40 implementations of the address space dependent functions for the virtual memory
18 and the physical memory by default, for a refe 41 and the physical memory by default, for a reference and convenient use.
19 42
20 43
21 Can I simply monitor page granularity? 44 Can I simply monitor page granularity?
22 ====================================== 45 ======================================
23 46
24 Yes. You can do so by setting the ``min_nr_re 47 Yes. You can do so by setting the ``min_nr_regions`` attribute higher than the
25 working set size divided by the page size. Be 48 working set size divided by the page size. Because the monitoring target
26 regions size is forced to be ``>=page size``, 49 regions size is forced to be ``>=page size``, the region split will make no
27 effect. 50 effect.
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