Re: Random file I/O regressions in 2.6

Ram Pai
2004-05-11T20:52:28Z

[-- Attachment #1: Type: text/plain, Size: 1634 bytes --]

On Mon, 2004-05-10 at 16:07, Andrew Morton wrote:
> Ram Pai <linuxram@us.ibm.com> wrote:
> >
> > I am nervous about this change. You are totally getting rid of
> > lazy-readahead and that was the optimization which gave the best
> > possible boost in performance.
>
> Because it disabled the large readahead outside the area which the app is
> reading. But it's still reading too much.

> > Let me see how this patch does with a DSS benchmark.
>
> That was not a real patch. More work is surely needed to get that right.
>
> > In the normal large random workload this extra page would have
> > compesated for all the wasted readaheads.
>
> I disagree that 64k is "normal"!
>
> > However in the case of
> > sysbench with Andrew's ra-copy patch the readahead calculation is not
> > happening quiet right. Is it worth trying to get a marginal gain
> > with sysbench at the cost of getting a big hit on DSS benchmarks,
> > aio-tests,iozone and probably others. Or am I making an unsubstantiated
> > claim? I will get back with results.
>
> It shouldn't hurt at all - the app does a seek, we perform the
> correctly-sized read.

Looks like you are right on all counts! I did some modifications to your
patch and did a preliminary run with my user-level simulator. With these
changes I am able to get rid of that extra page. Also code looks much
simpler and adapts well to sequential and random patterns.

However I have to run this under some benchmarks and see how it fares.
Its a pre-alpha level patch.

Can you take a quick look at the changes and see if you like it? I am
sure you won't consider these changes a hack ;)

RP

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diff -urNp linux-2.6.6/mm/readahead.c linux-2.6.6.new/mm/readahead.c
--- linux-2.6.6/mm/readahead.c 2004-05-09 19:32:00.000000000 -0700
+++ linux-2.6.6.new/mm/readahead.c 2004-05-11 20:26:51.288797696 -0700
@@ -353,7 +353,7 @@ page_cache_readahead(struct address_spac
unsigned orig_next_size;
unsigned actual;
int first_access=0;
- unsigned long preoffset=0;
+ unsigned long average=0;

/*
* Here we detect the case where the application is performing
@@ -394,10 +394,17 @@ page_cache_readahead(struct address_spac
if (ra->serial_cnt <= (max * 2))
ra->serial_cnt++;
} else {
- ra->average = (ra->average + ra->serial_cnt) / 2;
+ /* to avoid rounding errors, ensure that 'average'
+ * tends towards the value of ra->serial_cnt.
+ */
+ if(ra->average > ra->serial_cnt) {
+ average = ra->average - 1;
+ } else {
+ average = ra->average + 1;
+ }
+ ra->average = (average + ra->serial_cnt) / 2;
ra->serial_cnt = 1;
}
- preoffset = ra->prev_page;
ra->prev_page = offset;

if (offset >= ra->start && offset <= (ra->start + ra->size)) {
@@ -457,18 +464,14 @@ do_io:
* ahead window and get some I/O underway for the new
* current window.
*/
- if (!first_access && preoffset >= ra->start &&
- preoffset < (ra->start + ra->size)) {
- /* Heuristic: If 'n' pages were
- * accessed in the current window, there
- * is a high probability that around 'n' pages
- * shall be used in the next current window.
- *
- * To minimize lazy-readahead triggered
- * in the next current window, read in
- * an extra page.
+ if (!first_access) {
+ /* Heuristic: there is a high probability
+ * that around ra->average number of
+ * pages shall be accessed in the next
+ * current window.
*/
- ra->next_size = preoffset - ra->start + 2;
+ ra->next_size = (ra->average > max ?
+ max : ra->average);
}
ra->start = offset;
ra->size = ra->next_size;
@@ -492,21 +495,19 @@ do_io:
*/
if (ra->ahead_start == 0) {
/*
- * if the average io-size is less than maximum
+ * If the average io-size is more than maximum
* readahead size of the file the io pattern is
* sequential. Hence bring in the readahead window
- * immediately.
- * Else the i/o pattern is random. Bring
- * in the readahead window only if the last page of
- * the current window is accessed (lazy readahead).
+ * immediately.
+ * If the average io-size is less than maximum
+ * readahead size of the file the io pattern is
+ * random. Hence don't bother to readahead.
*/
- unsigned long average = ra->average;
-
+ average = ra->average;
if (ra->serial_cnt > average)
- average = (ra->serial_cnt + ra->average) / 2;
+ average = (ra->serial_cnt + ra->average + 1) / 2;

- if ((average >= max) || (offset == (ra->start +
- ra->size - 1))) {
+ if (average > max) {
ra->ahead_start = ra->start + ra->size;
ra->ahead_size = ra->next_size;
actual = do_page_cache_readahead(mapping, filp,