[Linux-cluster] Readhead Issues using cluster-1.01.00

[Velu Erwan]

Hi,
I've been playing with cluster-1.01.00 and I found the reading very slow.
I've been trying to setup "max_readahead" using gfs_tool and 
performances are unchanged.
So I've read the code and placed some printk everywhere ;o)

Results is : in opts_file.c when you make a gfs_read you have the 
following code (this is right for both buffered and directio reads)

        if (gfs_is_jdata(ip) ||
            (gfs_is_stuffed(ip) && !test_bit(GIF_PAGED, &ip->i_flags)))
                count = do_read_readi(file, buf, size, offset);
        else
                count = generic_file_read(file, buf, size, offset);

In my case, I always uses generic_file_read because all conditions are 
equal to 0.
I'm not a gfs expert so I don't know if it's normal or not to not use 
the do_read_readi call to read the fs.

I've watched how do_read_readi() works and after a few calls you call 
gfs_start_ra (from dio.c).
This sounds perfect because it defines "uint32_t max_ra = 
gfs_tune_get(sdp, gt_max_readahead) >> sdp->sd_sb.sb_bsize_shift;";
This uses the default max_readahead set.

Now the other case, what's about generic_file_read (defined in the usual 
linux tree)
Of course, it doesn't know about the value you set (max_readahead).
So after reading how it works, the file structure owns a f_ra member 
which is defined for handling the readahead aspects.
I though about forcing this value before the generic_file_read call.
Please found this patch attached.
When I set a default value to file->f_ra.ra_pages to 512, my 
performances are 3 times better ! I've bupped from 40MB/sec to 120MB/sec.
I'm now reaching the normal performance I was expected.

Comments about this patch:
1°) I don't know if it's the cleanest way to do it but it works, so It 
shows that the readahead aspect is not handled while using 
generic_file_read.
2°) I've used a default value that match my hardware but it could be 
cleaner to use the "gfs_tune_get(sdp, gt_max_readahead)" call.
2bis°) I don't know how to call it because it needs the gfs_lock 
structure and I don't know how to provide one (I didn't read enough code 
for that).
3°) I need a gfs guru to finish this patch with point 2 & 2bis if my 
patch sounds relevant

Erwan,
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