[Lustre-devel] readahead performance improvement

[Wallior, Julien]

> One of the problems with this change is that the first read of the file
> is at offset 0, but it should not invoke readahead.

In our case, we can do readahead starting at offset 0 because we assume
we'll be reading the whole file.

> The other problem is that this assumes the
> file striping is 1MB, but that isn't always the case.

I'm using the PTLRPC_MAX_BRW_SIZE for 1MB.

> Looking at the existing Lustre ll_readahead code I see:
>
>         /* Enlarge the RA window to encompass the full read */
>         if (bead != NULL && ras->ras_window_start +
>             ras->ras_window_len < bead->lrr_start + bead->lrr_count) {
>                 ras->ras_window_len = bead->lrr_start + bead->lrr_count -
>                         ras->ras_window_start;
>         }
>
> This is incorrect in my opinion.  The readahead window should be grown
> to the end of the first stripe boundary (usually 1MB, but it depends
> on striping), and then continue with aligned stripe sized reads (or
> multiples thereof to make full RPCs).  That can be determined by the
> readahead code correctly using the obd_extent_calc() method.

That would probably be a much better fix than mine. When we have a few
reading thread we get only 1MB IOs on the OSS. But when we have a lot
of thread (over 20), we see small IOs hitting the LUNs and that's what I'm
trying to prevent.

> I also see in two places the identical code:
>
>                 bead.lrr_start = *ppos >> CFS_PAGE_SHIFT;
>                 bead.lrr_count = (count + CFS_PAGE_SIZE - 1) >> CFS_PAGE_SHIFT;
>                 ll_ra_read_in(&bead);
>
> It also makes sense that this bead initialization is done in ll_ra_read_in(),
> and it should probably be renamed ll_ra_read_init().

That is probably a good idea. We are not using the path in ll_file_sendfile, so
I didn't change that.

> I'd be interested to see the patches, and I've CC'd lustre-devel for
> further discussion.

The patch is attached.

Julien

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