Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache only)

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 30 Mar 2015 18:49:06 -0400 Milosz Tanski <milosz@adfin.com> wrote:

> > A fincore+pread solution that blocks is simply unsafe
> > to use for us. We'll have to stay with the threadpool :-(.
> 
> We're getting data from a network filesystem Ceph in our case, but it
> could be pNFS. In many cases those filesystems have some kind
> hierarchy and it's not uncommon for us to se requests that take 20 to
> 25 milliseconds to complete. In this case the miss becomes very
> expensive. And it's not just that one requests experiences the slow
> down all the request being serviced by that (single) epoll thread
> experience head-of-line blocking because of one stalled request.
> 
> 10K request a second is a common load for many web services / video
> servers servings chunks of data. If we experience one miss a second,
> that 25 million stall will impact 250 other requests (all of them will
> have a 25ms latency tacked on).

I'd expect a fincore() which doesn't do SetPageReferenced() to be
orders of magnitude better than this.  A fincore() which does use
SetPageReferenced() will be in the "basically never happens" region -
it would take massive and artificial memory stress to trigger.