Re: [PATCH 4/6] vfs: add the RWF_HIPRI flag for preadv2/pwritev2

On Fri, Mar 04 2016, Christoph Hellwig wrote:

This adds a flag that tells the file system that this is a high priority
request for which it's worth to poll the hardware.  The flag is purely
advisory and can be ignored if not supported.

Here you say the flag is "advice".

 
+/* flags for preadv2/pwritev2: */
+#define RWF_HIPRI			0x00000001 /* high priority request, poll if possible */

This text makes it sound like a firm "request" ("if possible").

In the man page posted separately it says:

+.BR RWF_HIPRI " (since Linux 4.6)"
+High priority read/write.  Allows block based filesystems to use polling of the
+device, which provides lower latency, but may use additional ressources.  (Currently
+only usable on a file descriptor opened using the
+.BR O_DIRECT " flag)."

So now it "allows", which is different again.

The differences may be subtle, but consistency is nice.

Also in that man page fragment:

provides lower latency, but may use additional ressources

Is this a "latency vs throughput" trade-off, or something more subtle?
It would be nice to make the decision process as obvious as possible for
the developer considering the use of this flag.

(and s/ressources/resources/)

NeilBrown