On Fri, Jan 10, 2020 at 5:16 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote: > > From: SeongJae Park <sjpark@xxxxxxxxx> > > This commit adds a simple document for DAMON under > 'Documentation/admin-guide/mm/'. > > Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx> > --- > .../admin-guide/mm/data_access_monitor.rst | 235 ++++++++++++++++++ > Documentation/admin-guide/mm/index.rst | 1 + > 2 files changed, 236 insertions(+) > create mode 100644 Documentation/admin-guide/mm/data_access_monitor.rst > > diff --git a/Documentation/admin-guide/mm/data_access_monitor.rst b/Documentation/admin-guide/mm/data_access_monitor.rst > new file mode 100644 > index 000000000000..907a7af75f35 > --- /dev/null > +++ b/Documentation/admin-guide/mm/data_access_monitor.rst > @@ -0,0 +1,235 @@ > +.. _data_access_monitor: > + > +========================== > +DAMON: Data Access MONitor > +========================== > + > + > +Too Long; Don't Read > +==================== > + > +DAMON is a kernel module that allows users to monitor the actual memory access > +pattern of specific user-space processes. It aims to be 1) accurate enough to > +be useful for performance-centric domains, and 2) sufficiently light-weight so > +that it can be applied online. > + > +For the goals, DAMON utilizes its two core mechanisms, called region-based > +sampling and adaptive regions adjustment. The region-based sampling allows > +users to make their own trade-off between the quality and the overhead of the > +monitoring and set the upperbound of the monitoring overhead. Further, the > +adaptive regions adjustment mechanism makes DAMON to maximize the quality and > +minimize the overhead with its best efforts while preserving the users > +configured trade-off. > + > + > +Background > +========== > + > +For performance-centric analysis and optimizations of memory management schemes > +(either that of kernel space or user space), the actual data access pattern of > +the workloads is highly useful. The information need to be only reasonable > +rather than strictly correct, because some level of incorrectness can be > +handled in many performance-centric domains. It also need to be taken within > +reasonably short time with only light-weight overhead. > + > +Manually extracting such data is not easy and time consuming if the target > +workload is huge and complex, even for the developers of the programs. There > +are a range of tools and techniques developed for general memory access > +investigations, and some of those could be partially used for this purpose. > +However, most of those are not practical or unscalable, mainly because those > +are designed with no consideration about the trade-off between the accuracy of > +the output and the overhead. > + > +The memory access instrumentation techniques which is applied to many tools > +such as Intel PIN is essential for correctness required cases such as invalid > +memory access bug detections. However, those usually incur high overhead which > +is unacceptable for many of the performance-centric domains. Periodic access > +checks based on H/W or S/W access counting features (e.g., the Accessed bits of > +PTEs or the PG_Idle flags of pages) can dramatically decrease the overhead by > +forgiving some of the quality, compared to the instrumentation based > +techniques. The reduced quality is still reasonable for many of the domains, > +but the overhead can arbitrarily increase as the size of the target workload > +grows. Miniature-like static region based sampling can set the upperbound of > +the overhead, but it will now decrease the quality of the output as the size of > +the workload grows. > + > + > +Expected Use-cases > +================== > + > +A straightforward usecase of DAMON would be the program behavior analysis. > +With the DAMON output, users can confirm whether the program is running as > +intended or not. This will be useful for debuggings and tests of design > +points. > + > +The monitored results can also be useful for counting the dynamic working set > +size of workloads. For the administration of memory overcommitted systems or > +selection of the environments (e.g., containers providing different amount of > +memory) for your workloads, this will be useful. > + > +If you are a programmer, you can optimize your program by managing the memory > +based on the actual data access pattern. For example, you can identify the > +dynamic hotness of your data using DAMON and call ``mlock()`` to keep your hot > +data in DRAM, or call ``madvise()`` with ``MADV_PAGEOUT`` to proactively > +reclaim cold data. Even though your program is guaranteed to not encounter > +memory pressure, you can still improve the performance by applying the DAMON > +outputs for call of ``MADV_HUGEPAGE`` and ``MADV_NOHUGEPAGE``. More creative > +optimizations would be possible. Our evaluations of DAMON includes a > +straightforward optimization using the ``mlock()``. Please refer to the below > +Evaluation section for more detail. > + > +As DAMON incurs very low overhead, such optimizations can be applied not only > +offline, but also online. Also, there is no reason to limit such optimizations > +to the user space. Several parts of the kernel's memory management mechanisms > +could be also optimized using DAMON. The reclamation, the THP (de)promotion > +decisions, and the compaction would be such a candidates. > + > + > +Mechanisms of DAMON > +=================== > + > + > +Basic Access Check > +------------------ > + > +DAMON basically reports what pages are how frequently accessed. The report is > +passed to users in binary format via a ``result file`` which users can set it's > +path. Note that the frequency is not an absolute number of accesses, but a > +relative frequency among the pages of the target workloads. > + > +Users can also control the resolution of the reports by setting two time > +intervals, ``sampling interval`` and ``aggregation interval``. In detail, > +DAMON checks access to each page per ``sampling interval``, aggregates the > +results (counts the number of the accesses to each page), and reports the > +aggregated results per ``aggregation interval``. For the access check of each > +page, DAMON uses the Accessed bits of PTEs. > + > +This is thus similar to the previously mentioned periodic access checks based > +mechanisms, which overhead is increasing as the size of the target process > +grows. > + > + > +Region Based Sampling > +--------------------- > + > +To avoid the unbounded increase of the overhead, DAMON groups a number of > +adjacent pages that assumed to have same access frequencies into a region. As > +long as the assumption (pages in a region have 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 and clears > +its Accessed bit. After one more ``sampling interval``, DAMON reads the > +Accessed bit of the page and increases the access frequency of the region if > +the bit has set meanwhile. Therefore, the monitoring overhead is controllable > +by setting the number of regions. DAMON allows users to set the minimal and > +maximum number of regions for the trade-off. > + > +Except the assumption, this is almost same with the above-mentioned > +miniature-like static region based sampling. In other words, this scheme > +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 memory mapped address space of the process into the > +user-specified minimal number of regions. In this initial state, the > +assumption is normally not kept and thus the quality could be low. To keep the > +assumption as much as possible, DAMON adaptively merges and splits each region. > +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 regions if the total number of regions is smaller > +than the half of the user-specified maximum number of regions. > + > +In this way, DAMON provides its best-effort quality and minimal overhead while > +keeping the bounds users set for their trade-off. > + > + > +Applying Dynamic Memory Mappings > +-------------------------------- > + > +Only a number of small parts in the super-huge virtual address space of the > +processes is mapped to physical memory and accessed. Thus, tracking the > +unmapped address regions is just wasteful. However, tracking every memory > +mapping change might incur an overhead. For the reason, DAMON applies the > +dynamic memory mapping changes to the tracking regions only for each of an > +user-specified time interval (``regions update interval``). > + > + > +User Interface > +============== > + > +DAMON exports three files, ``attrs``, ``pids``, and ``monitor_on`` under its > +debugfs directory, ``<debugfs>/damon/``. > + > + > +Attributes > +---------- > + > +Users can read and write the ``sampling interval``, ``aggregation interval``, > +``regions update interval``, min/max number of regions, and the path to > +``result file`` by reading from and writing to the ``attrs`` file. For > +example, below commands set those values to 5 ms, 100 ms, 1,000 ms, 10, 1000, > +and ``/damon.data`` and check it again:: > + > + # cd <debugfs>/damon > + # echo 5000 100000 1000000 10 1000 /damon.data > attrs > + # cat attrs > + 5000 100000 1000000 10 1000 /damon.data > + > + > +Target PIDs > +----------- > + > +Users can read and write the pids of current monitoring target processes by > +reading from and writing to the `pids` file. For example, below commands set > +processes having pids 42 and 4242 as the processes to be monitored and check > +it again:: > + > + # cd <debugfs>/damon > + # echo 42 4242 > pids > + # cat pids > + 42 4242 > + > +Note that setting the pids doesn't starts the monitoring. > + > + > +Turning On/Off > +-------------- > + > +You can check current status, start and stop the monitoring by reading from and > +writing to the ``monitor_on`` file. Writing ``on`` to the file starts DAMON to > +monitor the target processes with the attributes. Writing ``off`` to the file > +stops DAMON. DAMON also stops if every target processes is be terminated. > +Below example commands turn on, off, and check status of DAMON:: > + > + # cd <debugfs>/damon > + # echo on > monitor_on > + # echo off > monitor_on > + # cat monitor_on > + off > + > +Please note that you cannot write to the ``attrs`` and ``pids`` files while the > +monitoring is turned on. If you write to the files while DAMON is running, > +``-EINVAL`` will be returned. > + > + > +User Space Wrapper > +------------------ > + > +DAMON has a shallow wrapper python script, ``/tools/damon/damn`` that provides > +more convenient interface. Note that it is only aimed to be used for minimal > +reference of the DAMON's raw interfaces and for debugging of the DAMON itself. > +Based on the debugfs interface, you can create another cool and more convenient > +user space tools. > + > + > +Quick Tutorial > +-------------- > + > +To test DAMON on your system, > + > +1. Ensure your kernel is built with CONFIG_DAMON turned on, and debugfs is > + mounted at ``/sys/kernel/debug/``. > +2. ``<your kernel source tree>/tools/damon/damn -h`` I think it would be helpful for the reader to provide an example of what they should expect to see here. > diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst > index 11db46448354..d3d0ba373eb6 100644 > --- a/Documentation/admin-guide/mm/index.rst > +++ b/Documentation/admin-guide/mm/index.rst > @@ -27,6 +27,7 @@ the Linux memory management. > > concepts > cma_debugfs > + data_access_monitor > hugetlbpage > idle_page_tracking > ksm > -- > 2.17.1 >
