Re: [PATCH 15/24] xfs: eagerly free shadow buffers to reduce CIL footprint

Dave Chinner
2019-08-05T23:34:37Z

On Mon, Aug 05, 2019 at 02:03:01PM -0400, Brian Foster wrote:
> On Thu, Aug 01, 2019 at 12:17:43PM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > The CIL can pin a lot of memory and effectively defines the lower
> > free memory boundary of operation for XFS. The way we hang onto
> > log item shadow buffers "just in case" effectively doubles the
> > memory footprint of the CIL for dubious reasons.
> >
> > That is, we hang onto the old shadow buffer in case the next time
> > we log the item it will fit into the shadow buffer and we won't have
> > to allocate a new one. However, we only ever tend to grow dirty
> > objects in the CIL through relogging, so once we've allocated a
> > larger buffer the old buffer we set as a shadow buffer will never
> > get reused as the amount we log never decreases until the item is
> > clean. And then for buffer items we free the log item and the shadow
> > buffers, anyway. Inode items will hold onto their shadow buffer
> > until they are reclaimed - this could double the inode's memory
> > footprint for it's lifetime...
> >
> > Hence we should just free the old log item buffer when we replace it
> > with a new shadow buffer rather than storing it for later use. It's
> > not useful, get rid of it as early as possible.
> >
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > ---
> > fs/xfs/xfs_log_cil.c | 7 +++----
> > 1 file changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> > index fa5602d0fd7f..1863a9bdf4a9 100644
> > --- a/fs/xfs/xfs_log_cil.c
> > +++ b/fs/xfs/xfs_log_cil.c
> > @@ -238,9 +238,7 @@ xfs_cil_prepare_item(
> > /*
> > * If there is no old LV, this is the first time we've seen the item in
> > * this CIL context and so we need to pin it. If we are replacing the
> > - * old_lv, then remove the space it accounts for and make it the shadow
> > - * buffer for later freeing. In both cases we are now switching to the
> > - * shadow buffer, so update the the pointer to it appropriately.
> > + * old_lv, then remove the space it accounts for and free it.
> > */
>
> The comment above xlog_cil_alloc_shadow_bufs() needs a similar update
> around how we handle the old buffer when the shadow buffer is used.

*nod*

>
> > if (!old_lv) {
> > if (lv->lv_item->li_ops->iop_pin)
> > @@ -251,7 +249,8 @@ xfs_cil_prepare_item(
> >
> > *diff_len -= old_lv->lv_bytes;
> > *diff_iovecs -= old_lv->lv_niovecs;
> > - lv->lv_item->li_lv_shadow = old_lv;
> > + kmem_free(old_lv);
> > + lv->lv_item->li_lv_shadow = NULL;
> > }
>
> So IIUC this is the case where we allocated a shadow buffer, the item
> was already pinned (so old_lv is still around) but we ended up using the
> shadow buffer for this relog. Instead of keeping the old buffer around
> as a new shadow, we toss it. That makes sense, but if the objective is
> to not leave dangling shadow buffers around as such, what about the case
> where we allocated a shadow buffer but didn't end up using it because
> old_lv was reusable? It looks like we still keep the shadow buffer
> around in that scenario with a similar lifetime as the swapout scenario
> this patch removes. Hm?

Of the top of my head, we shouldn't allocate a new shadow buffer in
that case (see xlog_cil_alloc_shadow_bufs()). i.e. we check up front
if the formatted size of the item will fit in the existing buffer,
and if it does we do not allocate a new shadow buffer as we just
reuse the existing one. SO we should only have to free a shadow
buffer when we switch them, not when we overwrite.

I'll recheck this, but I'm pretty sure overwrite won't leave a
shadow buffer around.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com