How can I do SEO of my elgg site ?

What do you mean?

https://elgg.org/plugins/1555823/releases/1.0.1#elgg-object-1748511
See the discussions/comments there for tips.

https://elgg.org/discussion/view/2620449/seo

https://elgg.org/discussion/view/2396236/seo-structured-data-making-elgg-sites-discoverable-by-google-etc

In Elgg 2.x there is something called as "Use simple cache" in Advance settings. I don't remember if this feature was there in 1.8.

Tip: Once Elgg 3 is released you may not get any further support for 1.8. why not upgrade your site to 2.x?

Try to disable 3rd party plugin one by one, flush the cache each time and run an upgrade after your flush. This way you can identify the plugin that is creating the problem.

Hint: On viewing the source code I found this:

<!-- Generated on Friday 23rd of November 2018 08:26:47 AM .--><!-- Static page served using Elgg-galliCache(1.1). Powered by Team Webgalli. -->

I guess galliCache might be causing the problem.

Thank you @Rohit Gupta

Without knowing what requires all the memory it's IMPOSSIBLE to give you any advice how you might be able to improve your server configuration.

You already posted the same question no long ago and I already gave you some hints what you could try to find out what might be wrong. Have you tried any of my suggestions at all? If you ask the same question again and again without trying for yourself what has been suggested you won't get it any improvements ever. It's simply not possible to give you this one advice that will fix the problem for sure because there could be many reasons for the out-of-memory issue (plugin requires too much memory or at least more than memory_limit in .htaccess, keeping connections of mysql alive with a too hight maximum number of connections or some bad settings in mysql configuration not suitable for the amount of ram of your server etc. etc.).

You need to monitor your server and test out memory consumption at least for a few hours or days to find out when the memory consumption gets critical.

Or hire someone with good knowledge of server administration or administrating Elgg sites (or knowledge about both) to help you with improving your server configuration. For example, you could ask for (paid) help here in the Professional services group and you might get contacted with someone who takes such jobs. Even if it will cost you it might be your only chance to get it fixed if you are not ready to learn more about server administration on your own (something you can't do in just a few hours or days for sure!).