Re: [Qemu-block] [PATCH] docs: document how to use the l2-cache-entry-size parameter

[Alberto Garcia]

On Thu 22 Feb 2018 03:17:57 PM CET, Kevin Wolf wrote:
>> > While we're at it, would l2-cache-entry-size = MIN(cluster_size,
>> > 64k) make sense as a default?
>> 
>> Any reason why you choose 64k, or is it because it's the default
>> cluster size?
>> 
>> In general I'd be cautious to reduce the default entry size
>> unconditionally because with rotating HDDs it may even have a
>> negative (although slight) effect on performance. But the larger the
>> cluster, the larger the area mapped by L2 entries, so we need less
>> metadata and it makes more sense to read less in general.
>> 
>> In summary: while I think it's probably a good idea, it would be good
>> to make some tests before changing the default.
>
> The exact value of 64k is more or less because it's the default
> cluster size, yes. Not changing anything for the default cluster size
> makes it a bit easier to justify.
>
> What I really want to fix is the 2 MB entry size with the maximum
> cluster size, because that's just unreasonably large. It's not
> completely clear what the ideal size is, but when we increased the
> default cluster size from 4k, the optimal values (on rotating hard
> disks back then) were 64k or 128k. So I assume that it's somewhere
> around these sizes that unnecessary I/O starts to hurt more than
> reading one big chunk instead of two smaller ones helps.
>
> Of course, guest I/O a few years ago and metadata I/O today aren't
> exactly the same thing, so I agree that a change should be measured
> first. But 64k-128k feels right as an educated guess where to start.

Yes, I agree.

>> > So should we default to this minimum on the grounds that for most
>> > people, refcounts blocks are probably only accessed sequentially in
>> > practice? The remaining memory of the total cache size seems to
>> > help the average case more if it's added to the L2 cache instead.
>> 
>> That sounds like a good idea. We should double check that the minimum
>> is indeed the required minimum, and run some tests, but otherwise I'm
>> all for it.
>> 
>> I think I can take care of this if you want.
>
> That would be good.
>
> I'm pretty confident that the minimum is enough because it's also the
> default for 64k clusters. Maybe it's too high if I miscalculated back
> then, but I haven't seen a crash report related to running out of
> refcount block cache entries.

Ok, I'll take a look.

Berto