So how I do with the problems of fragmented memory or buffer leak?

Thanks.

Open a TAC case and Cisco will help you resolve it (usually with IOS).

Normally fragmented memory or buffer leaks shouldn't been seen in typical operations. If there's a bug, which requires scheduled reloads until it's resolved, reload schedule would need to be before the bug causes your router to hang. (I.e. a general reload so often rule wouldn't apply.)

Beside reloads, the later IOSs have some additional optional features, I believe, to try to keep the router running. I think I've read there an optional buffer leak detector(?) and there also ERM and EEM to monitor the router and can try to inform the IOS to free RAM before it causes issues.

My experience with IOS it doesn't often deal well with low memory situations even when there's no bugs, so you also want to insure router has ample memory for expected production usage.

Normally, the 3800 series router takes between 7-10 minutes to reload.

If you want to reload the router with the least amount of time, I recommend you consider the "warm upgrade" and the "warm reload" which should shorten the upgrade and reload time by 60%.

Warm Upgrade info:

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t11/feature/guide/gtwarmup.html

Warm Reload info:

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t11/feature/guide/gtwarmup.html

Does this information help?

If we are to talk about this, what IOS version you have running?

Also does someone know if it s possible to restart some services on cisco routers, smthg similar with Linux (eg. I want to restart the routing module on a 2821 model).

Cisco will recommend that you use the latest IOS because it fixes bugs or new features/functions are introduced by it.

Depends on what features you need running.