Re: [PATCH v3 01/23] multifd: Delete useless operation

[Juan Quintela <quintela@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> * Juan Quintela (quintela@redhat.com) wrote:
>> We are divining by page_size to multiply again in the only use.
>              ^--- typo
>> Once there, impreve the comments.
>                   ^--- typo
>> 
>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>
> OK, with the typo's fixed:

Thanks.

> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>
> but, could you also explain the  x 2 (that's no worse than the current
> code); is this defined somewhere in zlib?  I thought there was a routine
> that told you the worst case?

Nowhere.

There are pathological cases where it can be worse.  Not clear at all
how much (ok, for zlib it appears that it is on the order of dozen of
bytes, because it marks it as uncompressed on the worst possible case),
For zstd, there is not a clear/fast answer when you google.

As this buffer is held for the whole migration, it is one for thread,
this looked safe to me.  Notice that we are compressing 128 pages at a
time, so for it not to compress anything looks very pathological.

But as one says, better safe than sorry.

If anyone that knows more about zlib/zstd give me different values, I
will change that in an additional patch.

Later, Juan.