what is the procedure for switching from Predis to PhpRedis?

Nothing, just change your config and flush the cache of you have any issues.

Should we add this to wp-config.php and then flush the cache?

define( 'WP_REDIS_CLIENT', 'pecl' );

Trying to figure this out myself at the moment.

@whereskarlo: Correct, you got it!

It didn’t work. However, upgrading to PHP 7.3 took care of it and I now have PhpRedis. No edit to wp-config was necessary in the end.

@whereskarlo I assume you must have that define( 'WP_REDIS_CLIENT', 'pecl' ); in the wp-config still.

I’m running under PHP7.3 and it doesn’t automatically switch over,
If I look at object-cache.php if WP_REDIS_CLIENT the default is still predis if the WP_REDIS_CLIENT is not defined.

        $client = defined('WP_REDIS_CLIENT') ? WP_REDIS_CLIENT : null;

        if (class_exists('Redis') && strcasecmp('predis', $client) !== 0) {
            $client = defined('HHVM_VERSION') ? 'hhvm' : 'pecl';
        } else {
            $client = 'predis';
        }

in any case it does not switch over to phpredis in my case even with the suggested define( 'WP_REDIS_CLIENT', 'pecl' ); added to wp-config

You probably added the constant to late in your config.

no, I just haven’t installed the PhpRedis pecl.
Not clear whether it is urgent to do so at this stage or whether it has any immediate benefits.

@jonathanmoorebcsorg using the native module has much better performance. I would recommend using it, especially since it’s really easy to do.

Ubuntu/Debian
apt-get install -y php-redis

Centos/Redhat
yum install -y php-pecl-redis

Alpine Linux in Docker (because I just had to set this up…)

RUN set -xe \
    && apk add --no-cache --update --virtual .phpize-deps $PHPIZE_DEPS \
    && pecl install -o -f redis \
    && docker-php-ext-enable redis \
    && apk del .phpize-deps