Re: [PATCH v5 7/8] multifd: Add multifd-zstd-level parameter

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Markus Armbruster (armbru@redhat.com) wrote:
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
> 
> > * Juan Quintela (quintela@redhat.com) wrote:
> >> Signed-off-by: Juan Quintela <quintela@redhat.com>
> >> ---
> >>  migration/migration.c | 15 +++++++++++++++
> >>  monitor/hmp-cmds.c    |  4 ++++
> >>  qapi/migration.json   | 29 ++++++++++++++++++++++++++---
> >>  3 files changed, 45 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/migration/migration.c b/migration/migration.c
> >> index 3b081e8147..b690500545 100644
> >> --- a/migration/migration.c
> >> +++ b/migration/migration.c
> >> @@ -91,6 +91,8 @@
> >>  #define DEFAULT_MIGRATE_MULTIFD_METHOD MULTIFD_METHOD_NONE
> >>  /*0: means nocompress, 1: best speed, ... 9: best compress ratio */
> >>  #define DEFAULT_MIGRATE_MULTIFD_ZLIB_LEVEL 1
> >> +/* 0: means nocompress, 1: best speed, ... 20: best compress ratio */
> >> +#define DEFAULT_MIGRATE_MULTIFD_ZSTD_LEVEL 1
> >>  
> >>  /* Background transfer rate for postcopy, 0 means unlimited, note
> >>   * that page requests can still exceed this limit.
> >> @@ -805,6 +807,8 @@ MigrationParameters *qmp_query_migrate_parameters(Error **errp)
> >>      params->multifd_method = s->parameters.multifd_method;
> >>      params->has_multifd_zlib_level = true;
> >>      params->multifd_zlib_level = s->parameters.multifd_zlib_level;
> >> +    params->has_multifd_zstd_level = true;
> >> +    params->multifd_zstd_level = s->parameters.multifd_zstd_level;
> >
> > Do we really want different 'multifd_...._level's or just one
> > 'multifd_compress_level' - or even just reuse the existing
> > 'compress-level' parameter.
> 
> compress-level, multifd-zlib-level, and multifd-zstd-level apply
> "normal" live migration compression, multifd zlib live migration
> compression, and multifd zstd live migration compression, respectively.
> 
> Any live migration can only use one of the three compressions.
> 
> Correct?

Right.

> > The only tricky thing about combining them is how to handle
> > the difference in allowed ranges;  When would the right time be
> > to check it?
> >
> > Markus/Eric: Any idea?
> 
> To have an informed opinion, I'd have to dig through the migration
> code.

The tricky part I see is validating settings/parameters;
when someone tries to set a parameter migrate_params_check gets called
and has checks like:

    if (params->has_compress_level &&
        (params->compress_level > 9)) {
        error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "compress_level",
                   "is invalid, it should be in the range of 0 to 9");
        return false;
    }


now that's nice because you error when you set the parameter rather
than later when you try and start a migration; the downside now
though is you get more complex interaction between parameters and
capabilities - so for example if you set the multifd-compression type
parameter to 'zstd' and *then* set the single compression level
it'll be fine taking '20' as a compresison level, but if you were
to set the compression level to 20 and then set the type to 'zstd'
it might error because with zlib you can't have 20.


> Documentation of admissible range will become a bit awkward, too.
> 
> Too many migration parameters...

Nod; which why I was trying to make it 1.

Dave

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK