Hello Andrew, Thank you for great questions! On Sun, 10 Oct 2021 15:01:40 -0700 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Fri, 8 Oct 2021 09:45:06 +0000 SeongJae Park <sj@xxxxxxxxxx> wrote: > > > The user space can get the monitoring results via the 'damon_aggregated' > > tracepoint event. For simplicity and brevity, the tracepoint events > > have some duplicated information such as 'target_id' and 'nr_regions', > > though. As a result, its size is greater than really needed. Also, > > dealing with the tracepoint could be complex for some simple use cases. > > To provide a way for getting more efficient and simple monitoring > > results to user space, this commit implements 'recording' feature in > > 'damon-dbgfs'. > > > > The feature is exported to the user space via a new debugfs file named > > 'record', which is located in '<debugfs>/damon/' directory. The file > > allows users to record monitored access patterns in a regular binary > > file in a simple format. > > Binary files are troublesome. > > Is the format of this file documented anywhere? No. I intended the Python script in the following patch[1] and the user space tool[2] to be used as such documents. I will write up one before the next spin. [1] https://lore.kernel.org/linux-mm/20211008094509.16179-3-sj@xxxxxxxxxx/ [2] https://github.com/awslabs/damo/blob/v0.0.5/_damon_result.py#L38 > > I assume that the file's contents will have different representations > depending on host endianness and word size and I further assume that > the provided python script won't handle this very well? You're right. I will make the script properly handle the cases in the next spin. > > > The recorded results are first written in an > > in-memory buffer and flushed to a file in batch. Users can get and set > > the size of the buffer and the path to the result file by reading from > > and writing to the 'record' file. For example, below commands set the > > buffer to be 4 KiB and the result to be saved in '/damon.data'. > > > With a simple test workload[1], recording the tracepoint event using > > 'perf-record' results in 1.7 MiB 'perf.data' file. When the access > > pattern is recorded via this feature, the size is reduced to 264 KiB. > > Also, the resulting record file is simple enough to be manipulated by a > > small (100 lines of code) python script which will be introduced by a > > following commit ("selftests/damon: Test recording feature"). > > How useful and important is this? I mean, is it tremendously better or > is it a little bit nice to have? A description of the overall benefit > to DAMON users would be useful in helping others to understand the > benefit of this change. Very good point. Expected benefits are 1) better access pattern recording space efficiency and 2) making it not depend on tracepoints. Nevertheless, I realized the importance of the benefit is not well quantified, thanks to this question. I will make it clear in the next spin. Nevertheless, this feature is not critical for now. I will deprioritize this patchset and post other patchesets in DAMON development tree, namely 1) support of physical address space monitoring and 2) DAMON-based proactive reclamation first. Thanks, SJ
