This instructs the kernel that it can release memory used by mmap'd
TSM files when they are not actively being used. It the mappings are
use, the kernel will fault the pages back in. On linux, this causes
RES memory to drop immediately when run.
This may be causing slow restart times for systems with many large TSM files.
What I believe is happening at startup in these cases is that multiple goroutines
are started to load each TSM file concurrently. The kernel appears to serialize
mmap calls from the same process so all of the goroutines end up getting blocked
on the actual mmap system call. MAP_POPULATE instruct the kernel to pre-fault the
page table for the files and triggers read-ahead of the pages. For larger, 2GB files,
this makes the mmap call more expensive and slower. When there are many of these files
and calls it is possible to fill all available memory with pagecache. In this case,
the OS will end up pre-faulting pages from one file and have to remove pages that it just
loaded from another files causing slowness. MAP_POPULATE may also be cause much more data
to be pre-faulted than necessary. To load a file, we just need to scan the index at the end
of the file. MAP_POPULATE is likely causing the whole file to be loaded when it won't actually
be accessed for a while (or at all).
Might fix issue #5311.
This has a few changes in it (unfortuantely). The main change is to run compactions
concurrently. While implementing this, a few query and performance bugs showed up that
are also fixed by this commit.
Dane Strandboge
Sam Arnold
Edd Robinson
Ben Johnson
Jason Wilder