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Avast Antivirus and µTorrent is a bad mix in regards to youre speeds


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(I was typing quite a long post here about my really strange upload/download speeds, but before I could finish it up, I discovered what was causing it)

so instead, Ill post here a heads up for those of you who use µTorrent and the Avast Antivirus program.

what happens is that after a while downloading, mainly with many active torrents, the download and upload speeds start to act strange. the download and upload seems to be linked yielding equal download and upload speeds, and then it starts where the upload suddenly stops for a few seconds before it resumes. now when the upload stops the download speeds skyrockets to only drop down again when the upload kicks in again, leaving a lovely spike in speed loggers.

this start and stop seems to happen at a regular pace on every minute.


to me it seems like it is Avast's P2P shield which is designed to scan in/outgoing transmissions in known P2P clients and protect the users from downloading malware, trojans and viruses is the culprit of this odd behaviour, as Ive witnessed that Avast's scanner gobs up to 100% cpu usage the moment the upload flatlines and stops the heavy cpu usage when uploads kick in again.

Stopping Avast's P2P shield and this behaviour stops right away and as a little bonus my downloads went up. :)

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Hmm. How doest Avast! detect P2P applications ? If so, you might wanna go to their site\forum\any way to contact them and tell them about this issue, so uTorrent would get added as a known p2p application.

I've already heard about one AV adding uTorrent to his known programs list in order to prevent issues.

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I think Avast detects P2P program is by task/exe name, and µTorrent is allready in Avast's list of known P2P programs. how they listen to the traffic I dont know, most likely by acting as a firewall but it seems to me that Avast does things wrong. it as if Avast tries to save up buffers with data it can scan and when this buffer is filled up Avast stops the data flow while it scans what it allready had recieved, giving this odd behaviour in upload/download speeds. it seems to me that this isnt any problem running a single torrent or two, but when I try to download several torrents this effect kicks in. it might even be that it handles poorly hundreds of active connections, as Ive set uTorrent to use a global limit of 800 active, 200 active per torrent and max 50 half-open connections (on Win2K here).

anyhows, Ill contact Avast and ask them about this behaviour


are you shure you dont mean manually make Avast ignore µTorrent?

as said above, µTorrent is in this list of known P2P list, and it is guarded by default, but this shield doesnt work properly as it both limits the downloadspeed and leads to chocking the uploadspeed on a regular basis. its simple to make Avast ignore uTorrent in the P2P shield, but for me as I dont use any other P2P software, its useless doing that.. ;)

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well, Ive sent a mail to Avast asking about this behaviour and today, I got the reply.


the problem is probably caused by the nature of p2p shield. It scans the

downloads every time the file is closed (as an indication that a write

operation is completed), and your bittorrent client probably closes the

downloaded file after every chunk. Since it is a large file (hundreds of

megabytes) it takes a few seconds to scan it - especially if it is

compressed or ISO image, avast tries to unpack (which might frequently fail,

until the file is downloaded in its entirety). During the scan the file is

locked and hence download will stop temporarily.

For you network and CPU speeds the P2P shield is not working correctly with

uTorrent. :-( All I can suggest is to uncheck your bittorrent client in the

P2P configuration and rely either on the Standard Shield (if you have it

enabled) or configure uTorrent to scan the downloads after they are finished

with ashQuick.exe - if that is an option.


Lukas Rypacek

software developer

Alwil Software


well, its nice to see that one of Avast's programmers took his time to reply to my question and not just some lowlife tech-support junkie with their usual brainless mumbo jumbo that doesnt make any sense.. ;)

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  • 6 years later...

"diskio.flush_files: Enabling this option causes µTorrent to close file handles every minute. It helps to reduce the effect of Windows managing the system cache badly for some people and causing apparent "memory leaks."

Based on your graphing it looks like you may not have high enough up/down to have that issue. p2p shield probably aint a huge deal vs realtime though, either way you can get screwed :P

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