Author: ken
Date: Fri Feb 27 02:44:12 2015
New Revision: 279346
URL: https://svnweb.freebsd.org/changeset/base/279346
Log:
Remove an obsolete comment in devstat(3) about the accuracy of the
milliseconds per transaction (DSM_MS_PER_TRANSACTION) calculation.
The comment was accurate many years ago when the kernel didn't
record I/O times on a per-I/O basis, but now that we do collect
that information in most areas, it isn't correct.
The milliseconds per transaction values are correct, assuming the
I/O duration has been recorded.
Sponsored by: Spectra Logic
MFC after: 3 days
Modified:
head/lib/libdevstat/devstat.c
Modified: head/lib/libdevstat/devstat.c
==============================================================================
--- head/lib/libdevstat/devstat.c Fri Feb 27 02:21:52 2015
(r279345)
+++ head/lib/libdevstat/devstat.c Fri Feb 27 02:44:12 2015
(r279346)
@@ -1487,22 +1487,9 @@ devstat_compute_statistics(struct devsta
*destld = 0.0;
break;
/*
- * This calculation is somewhat bogus. It simply divides
- * the elapsed time by the total number of transactions
- * completed. While that does give the caller a good
- * picture of the average rate of transaction completion,
- * it doesn't necessarily give the caller a good view of
- * how long transactions took to complete on average.
- * Those two numbers will be different for a device that
- * can handle more than one transaction at a time. e.g.
- * SCSI disks doing tagged queueing.
- *
- * The only way to accurately determine the real average
- * time per transaction would be to compute and store the
- * time on a per-transaction basis. That currently isn't
- * done in the kernel, and would only be desireable if it
- * could be implemented in a somewhat non-intrusive and high
- * performance way.
+ * Some devstat callers update the duration and some don't.
+ * So this will only be accurate if they provide the
+ * duration.
*/
case DSM_MS_PER_TRANSACTION:
if (totaltransfers > 0) {
@@ -1512,11 +1499,6 @@ devstat_compute_statistics(struct devsta
} else
*destld = 0.0;
break;
- /*
- * As above, these next two really only give the average
- * rate of completion for read and write transactions, not
- * the average time the transaction took to complete.
- */
case DSM_MS_PER_TRANSACTION_READ:
if (totaltransfersread > 0) {
*destld = totaldurationread;
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