On Wed, 15 Jan 2014 12:47:35 +0400 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> wrote:
> On 01/15/2014 02:14 AM, Andrew Morton wrote:
> > On Tue, 14 Jan 2014 11:23:30 +0400 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> wrote:
> >
> >> On 01/14/2014 03:05 AM, Andrew Morton wrote:
> >>> That being said, I think I'll schedule this patch as-is for 3.14. Can
> >>> you please take a look at implementing the simpler approach, send me
> >>> something for 3.15-rc1?
> >> IMHO the simpler approach (Glauber's patch) is not suitable as is,
> >> because it, in fact, neglects the notion of batch_size when doing low
> >> prio scans, because it calls ->scan() for < batch_size objects even if
> >> the slab has >= batch_size objects while AFAIU it should accumulate a
> >> sufficient number of objects to scan in nr_deferred instead.
> > Well. If you mean that when nr-objects=large and batch_size=32 and
> > total_scan=33, the patched code will scan 32 objects and then 1 object
> > then yes, that should be fixed.
>
> I mean if nr_objects=large and batch_size=32 and shrink_slab() is called
> 8 times with total_scan=4, we can either call ->scan() 8 times with
> nr_to_scan=4 (Glauber's patch) or call it only once with nr_to_scan=32
> (that's how it works now). Frankly, after a bit of thinking I am
> starting to doubt that this can affect performance at all provided the
> shrinker is implemented in a sane way, because as you've mentioned
> shrink_slab() is already a slow path. It seems I misunderstood the
> purpose of batch_size initially: I though we need it to limit the number
> of calls to ->scan(), but now I guess the only purpose of it is limiting
> the number of objects scanned in one pass to avoid latency issues.
Actually, the intent of batching is to limit the number of calls to
->scan(). At least, that was the intent when I wrote it! This is a
good principle and we should keep doing it. If we're going to send the
CPU away to tread on a pile of cold cachelines, we should make sure
that it does a good amount of work while it's there.
> But
> then another question arises - why do you think the behavior you
> described above (scanning 32 and then 1 object if total_scan=33,
> batch_size=32) is bad?
Yes, it's a bit inefficient but it won't be too bad. What would be bad
would be to scan a very small number of objects and then to advance to
the next shrinker.
> In other words why can't we make the scan loop
> look like this:
>
> while (total_scan > 0) {
> unsigned long ret;
> unsigned long nr_to_scan = min(total_scan, batch_size);
>
> shrinkctl->nr_to_scan = nr_to_scan;
> ret = shrinker->scan_objects(shrinker, shrinkctl);
> if (ret == SHRINK_STOP)
> break;
> freed += ret;
>
> count_vm_events(SLABS_SCANNED, nr_to_scan);
> total_scan -= nr_to_scan;
>
> cond_resched();
> }
Well, if we come in here with total_scan=1 then we defeat the original
intent of the batching, don't we? We end up doing a lot of work just
to scan one object. So perhaps add something like
if (total_scan < batch_size && max_pass > batch_size)
skip the while loop
If we do this, total_scan will be accumulated into nr_deferred, up to
the point where the threshold is exceeded, yes?
All the arithmetic in there hurts my brain and I don't know what values
total_scan typically ends up with.
btw. all that trickery with delta and lru_pages desperately needs
documenting. What the heck is it intended to do??
We could avoid the "scan 32 then scan just 1" issue with something like
if (total_scan > batch_size)
total_scan %= batch_size;
before the loop. But I expect the effects of that will be unmeasurable
- on average the number of objects which are scanned in the final pass
of the loop will be batch_size/2, yes? That's still a decent amount.
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