>> If applications want more memory, they just take it back from the disk cache.
Q: If there is no swap configured, will a malloc() then take away clean page-cache pages ? Or does that happen only on page-in ?
In general, no and it will happen when there is something actually written to the page (which will cause a page fault and the kernel will have to somehow materialize the page). This works the same way regardless of how /proc/sys/vm/overcommit_memory is configured, the setting only affects how kernel tracks how much memory it is going to need in the future. (Obviously if we talk about malloc() this is a slight over-simplification as most malloc() implementations will write some kind of book-keeping structure and thus dirty some of the allocated pages)
Whether swap is available is more or less irrelevant for this behavior. The only thing that swap changes is that kernel is then able to “clean” dirty anonymous pages by writing them out to swap.
Fwiw without swap there isn't really any paging in or out (yes mmapped files technically still can but they are basically a special cased type of swap) so your question is hard to parse in this context. The disk cache is all about using unallocated memory and an allocation will reduce it. Paging is irrelevant here.
Btw you should always enable swap. Without it you force all unused but allocated memory to live on physical RAM. Why would you want to do this? There's absolutely no benchmarks that show better performance with no swap. In fact it's almost always the opposite. Add some swap. Enjoy the performance boost!
I would say that for any modern unix implementation mmaped pages are quite significant, as all the read-only copies of libc code, other shared libraries and various mmaped data files (iconv tables, locales, terminfo, gettext catalogs…) are not exactly small.
dfox|2 years ago
Whether swap is available is more or less irrelevant for this behavior. The only thing that swap changes is that kernel is then able to “clean” dirty anonymous pages by writing them out to swap.
AnotherGoodName|2 years ago
Fwiw without swap there isn't really any paging in or out (yes mmapped files technically still can but they are basically a special cased type of swap) so your question is hard to parse in this context. The disk cache is all about using unallocated memory and an allocation will reduce it. Paging is irrelevant here.
Btw you should always enable swap. Without it you force all unused but allocated memory to live on physical RAM. Why would you want to do this? There's absolutely no benchmarks that show better performance with no swap. In fact it's almost always the opposite. Add some swap. Enjoy the performance boost!
https://haydenjames.io/linux-performance-almost-always-add-s...
dfox|2 years ago