Re: RAID-0 read perf. decrease after 2.4.20

From: Marcelo Tosatti
Date: Mon Dec 08 2003 - 08:06:30 EST



On Fri, 21 Nov 2003, Mikael Johansson wrote:

>
> Hello Marcelo and All!

Hi Mikael,

>
> On Fri, 21 Nov 2003, Marcelo Tosatti wrote:
>
> > There have been no significant changes in the RAID driver in 2.4.21, so I
> > suspect the cause for the slowdowns might the changes to the IO scheduler
> > (ll_rw_blk.c) or VM changes.
> >
> > Isolating the -pre which the slowdown starts helps a lot.
>
> I tested a few kernels to pin point the where tha changes occured. It
> turned out that both 2.4.20 and 2.4.21-pre1 have bad read performance. The
> 2.4.20-ac's show good read speed. I tested it on two machines (different
> from yesterday). I also checked the VIA chipset drivers version; that's
> not the reason for the differences.
>
> Athlon XP 2400+, 2.09 GHz, 1.5 GB RAM, 2*160 GB Maxtor Maxtor 6Y080L0
> VIA write read
> 2.4.19 none 10,000 9,000 K/sec (chipset not supported)
> 2.4.20 3.35 73,000 88,000
> 2.4.20-ac1 3.35-ac 70,000 135,000
> 2.4.20-ac2 3.35-ac 71,000 140,000
> 2.4.21-pre1 3.35-ac 71,000 79,000
> 2.4.21-pre3 3.35-ac 71,000 79,000
>
> Athlon 1.4 GHz, 1.5 GB RAM, 2*80 GB IC35L040AVER07-0 (IBM, I think)
> 2.4.13-ac8 ? 49,000 44,000 K/sec
> 2.4.19 3.34 53,000 41,000
> 2.4.20-ac2 3.35-ac 50,000 69,000
> 2.4.21-pre1 3.35-ac 53,000 46,000
>
> So there was apparently something very right with the 2.4.20-ac's. It
> would be nice to get it back :-)

2.4.20-aa included rmap and some VM modifications most notably
"drop_behind()" logic which I believe should be the reason for the huge
read speedups. Can you please try it? Against 2.4.23.

--- mm/filemap.c 2003-12-08 10:26:58.000000000 -0200
+++ mm/filemap.c.orig 2003-12-08 10:24:57.000000000 -0200
@@ -1055,56 +1055,6 @@
return page;
}

-
- /* We combine this with read-ahead to deactivate pages when we
- * think there's sequential IO going on. Note that this is
- * harmless since we don't actually evict the pages from memory
- * but just move them to the inactive list.
- *
- * TODO:
- * - make the readahead code smarter
- * - move readahead to the VMA level so we can do the same
- * trick with mmap()
- *
- * Rik van Riel, 2000
- */
-static void drop_behind(struct file * file, unsigned long index)
-{
- struct inode *inode = file->f_dentry->d_inode;
- struct address_space *mapping = inode->i_mapping;
- struct page *page;
- unsigned long start;
-
- /* Nothing to drop-behind if we're on the first page. */
- if (!index)
- return;
-
- if (index > file->f_rawin)
- start = index - file->f_rawin;
- else
- start = 0;
-
- /*
- * Go backwards from index-1 and drop all pages in the
- * readahead window. Since the readahead window may have
- * been increased since the last time we were called, we
- * stop when the page isn't there.
- */
- spin_lock(&pagemap_lru_lock);
- while (--index >= start) {
- struct page **hash = page_hash(mapping, index);
- spin_lock(&pagecache_lock);
- page = __find_page_nolock(mapping, index, *hash);
- spin_unlock(&pagecache_lock);
- if (!page || !PageActive(page))
- break;
- drop_page(page);
- }
- spin_unlock(&pagemap_lru_lock);
-}
-
-
-
/*
* Same as grab_cache_page, but do not wait if the page is unavailable.
* This is intended for speculative data generators, where the data can
@@ -1376,12 +1326,6 @@
if (filp->f_ramax > max_readahead)
filp->f_ramax = max_readahead;

- /*
- * Move the pages that have already been passed
- * to the inactive list.
- */
- drop_behind(filp, index);
-
#ifdef PROFILE_READAHEAD
profile_readahead((reada_ok == 2), filp);
#endif




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