On Sat, Apr 04, 2020 at 12:28:26PM -0400, hubcap@xxxxxxxxxx wrote: > As an aside, the page cache has been a blessing and a curse > for us. Since we started using it, small IO has improved > incredibly, but our max speed hits a plateau before it otherwise > would have. I think because of all the page size copies we have > to do to fill our 4 meg native buffers. I try to read about all > the new work going into the page cache in lwn, and make some > sense of the new code :-). One thing I remember is when > Christoph Lameter said "the page cache does not scale", but > I know the new work is focused on that. If anyone has any > thoughts about how we could make improvments on filling our > native buffers from the page cache (larger page sizes?), > feel free to offer any help... Umm, 4MB native buffers are ... really big ;-) I wasn't planning on going past PMD_SIZE (ie 2MB on x86) for the readahead large pages, but if a filesystem wants that, then I should change that plan. What I was planning for, but don't quite have an implementation nailed down for yet, is allowing filesystems to grow the readahead beyond that wanted by the generic code. Filesystems which implement compression frequently want blocks in the 256kB size range. It seems like OrangeFS would fit with that scheme, as long as I don't put a limit on what the filesystem asks for. So yes, I think within the next year, you should be able to tell the page cache to allocate 4MB pages. You will still need a fallback path for when memory is too fragmented to allocate new pages, but if you're using 4MB pages, then hopefully we'll be able to reclaim a clean 4MB pages from elsewhere in the page cache and supply you with a new one.
