Monitoring file reads for cache warming

Question

After adding more memory to my Linux machine I was tempted to use the cache more agressively.

The thing is: I boot up my machine, go to bathroom, come back and log in. It takes like half a minute to log in. Then I start firefox and again it takes some time. When I log out and back in, all these things happen within seconds, because the cache is warm then. So I thought: how can I teach my machine to put all these files into the cache while I am in the bathroom.

I looked into preload and found the results somewhat disappointing. The net cause of this seems to be that it only tracks mmap()ed files and not files which are read by means of read().

I suspect this is because mmap()ed files are easier to observe. When an application mmap()s a file it stays in memory for some time, whereas open()-read()-close() could all happen within a fraction of a second (correct me if I'm wrong).

There does not seem to be a simple way to get notified when a file is read from. There is inotifiy, but it requires registering directories and IIRC correctly, is not meant for monitoring entire filesystems.

Ideally I would like to have a device in /dev (like a tty) from which I can read the filenames from which some application reads from.

(1) Is something like this available ?

(2) Are there reasons why this is difficult ?

If I had information about read file accresses I could easily warm the cache during boot.

Answer 1

Regarding the caching of read system calls:

The read system call can be applied on a big number of socket types. So there is no caching logic for the read itself. However, if you do IO operations on a file system, the file system itself might support caching techniques, but this is depending on your file system.

This is different to mmap, because mmap is specifically made for mapping big files into memory and operating on them. If you do this, it might be worth to consider using mmap instead of read.

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Regarding the second part of your post, where you say that you would like some way to get the list of files that a defined application or process is reading from:

I don't know how you could get this kind of data in a device file in /dev. But by using System Tap it should be easy to create a kernel module which prints you all the arguments for read or open system calls that a defined process is doing into dmesg. Then you could add something like a predefined prefix to this output, so if you do dmesg | grep <prefix> you would get all the output from this kernel module in the same way as you describe you would like it from a /dev device.

Please note that this should not be used in production, but only for debugging or other non-production purposes.

This is a nice guide on System Tap basics, after learning it you should be able to create a System Tap script that only requires the name of an executable or the PID of a process as input. This could then print the arguments to all read or open system calls that come from a certain process or application into your dmesg:

https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/5/html/SystemTap_Beginners_Guide/