Need More Info
Last Updated: 08 Apr 2021 13:29 by ADMIN
Created on: 28 Mar 2021 10:34
Type: Bug Report
Memory leaks

Fiddler version: v5.0.20204.45441

If Fiddler is left for some time to work unattended then its memory usage is constantly growing.

I usually use Fiddler with some Autoresponder rules enabled and "Unmatched requests passthrough" option switched on. If left in this state for several hours an amount memory it uses could grow up to 10GB and even more which is quite annoying...

Autosave option is enabled and sessions are dumped every 5 minutes. So this behaviour is not because of session list is becoming large, it is being cleaned every 5 minutes.

Nick Iliev
Posted on: 08 Apr 2021 13:29

Hey Anton,


We are currently trying to reproduce the issue but so far to no avail. It is doubtful that the warning output (ClientHello record was 508 bytes long) is causing the memory leak. I will post here as soon as we manage to observe the memory leak or know more about a possible cause. Meanwhile, as Eric suggested, you could try investigating the Windows memory dump.


Nick Iliev
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Posted on: 02 Apr 2021 16:17

Nick, Thanks for your proposals. I tried to restrict amount of sessions to 500 but it does not affect anything. As I said before I have Autosave enabled so sessions list is cleared every 5 minutes.

Downloading big files seems also not relevant here because memory usage is growing even when no files downloaded at all. I have added the code snippet from the page you mentioned (to enable a streaming for the big files) but it also have no effect.

Eric, There are lots of records like this in the log:

HTTPSLint> Warning: ClientHello record was 508 bytes long. Some servers have problems with ClientHello's greater than 255 bytes.

I do not know if they are relevant to the leaks problem or not. Anyway log does not have the size of gigabytes definitely.

I use Fiddler "from the box" with no third party extensions or scripts.


Posted on: 02 Apr 2021 15:50

@Nick: The claim here is that the Session list is being cleared by the "AutoSave" option inside View > Tabs > AutoSave, which should be clearing the Session List (and thus the Sessions) on a regular basis.

You'd need to get a memory dump to understand what exactly is going wrong here-- either something else is keeping the Sessions alive (some other extension?), or there's some other data store growing in an unbounded manner (the most obvious possibility is the LOG tab, which will record the text of errors and whatnot. But it should take a very long time for that log to get to 10GB, unless you've got script or something else doing a lot of logging).

Nick Iliev
Posted on: 02 Apr 2021 05:41

Hello Anton,


You could try to limit the number of sessions that are kept by using theKeep XXX Sessions drop-down option in the toolbar. The default state is set to Keep All Sessions that are kept which is probably not the best option when you are going to capture for hours continuously. Another thing you could try is to drop large files via the technique shown here.

Please let us know if one or the other is solving the issue on your side.


Nick Iliev
Progress Telerik

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