On Mon, Jan 29 2024 at 5:12P -0500,
Dave Chinner <david@xxxxxxxxxxxxx> wrote:
On Mon, Jan 29, 2024 at 12:19:02PM -0500, Mike Snitzer wrote:
While I'm sure this legacy application would love to not have to
change its code at all, I think we can all agree that we need to just
focus on how best to advise applications that have mixed workloads
accomplish efficient mmap+read of both sequential and random.
To that end, I heard Dave clearly suggest 2 things:
1) update MADV/FADV_SEQUENTIAL to set file->f_ra.ra_pages to
bdi->io_pages, not bdi->ra_pages * 2
2) Have the application first issue MADV_SEQUENTIAL to convey that for
the following MADV_WILLNEED is for sequential file load (so it is
desirable to use larger ra_pages)
This overrides the default of bdi->ra_pages and _should_ provide the
required per-file duality of control for readahead, correct?
I just discovered MADV_POPULATE_READ - see my reply to Ming
up-thread about that. The applicaiton should use that instead of
MADV_WILLNEED because it gives cache population guarantees that
WILLNEED doesn't. Then we can look at optimising the performance of
MADV_POPULATE_READ (if needed) as there is constrained scope we can
optimise within in ways that we cannot do with WILLNEED.
Nice find! Given commit 4ca9b3859dac ("mm/madvise: introduce
MADV_POPULATE_(READ|WRITE) to prefault page tables"), I've cc'd David
Hildenbrand just so he's in the loop.
FYI, I proactively raised feedback and questions to the reporter of
this issue:
CONTEXT: madvise(WILLNEED) doesn't convey the nature of the access,
sequential vs random, just the range that may be accessed.