[lttng-dev] [-stable 3.8.1 performance regression] madvise POSIX_FADV_DONTNEED

[Jeff Moyer]

Mel Gorman <mgorman at suse.de> writes:

>> > I just tried replacing my sync_file_range()+fadvise() calls and instead
>> > pass the O_DIRECT flag to open(). Unfortunately, I must be doing
>> > something very wrong, because I get only 1/3rd of the throughput, and
>> > the page cache fills up. Any idea why ?
>> 
>> Since O_DIRECT does not seem to provide acceptable throughput, it may be
>> interesting to investigate other ways to lessen the latency impact of
>> the fadvise DONTNEED hint.
>> 
>
> There are cases where O_DIRECT falls back to buffered IO which is why you
> might have found that page cache was still filling up. There are a few
> reasons why this can happen but I would guess the common cause is that
> the range of pages being written was in the page cache already and could
> not be invalidated for some reason. I'm guessing this is the common case
> for page cache filling even with O_DIRECT but would not bet money on it
> as it's not a problem I investigated before.

Even when O_DIRECT falls back to buffered I/O for writes, it will
invalidate the page cache range described by the buffered I/O once it
completes.  For reads, the range is written out synchronously before the
direct I/O is issued.  Either way, you shouldn't see the page cache
filling up.

Switching to O_DIRECT often incurs a performance hit, especially if the
application does not submit more than one I/O at a time.  Remember,
you're not getting readahead, and you're not getting the benefit of the
writeback code submitting batches of I/O.

HTH,
Jeff