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[SOLVED] uTorrent reads excessively (it was read cache)


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I noticed my nice PC was kinda sluggish and I tracked the issue to uTorrent.

I downloaded a torrent, partial. The whole torrent was 45G, selected 5G. It was going at 1-2M download, 500K upload, quite steadily, but my disk had activity in the 50-70 MB/s, peaking at 100-120 (that's my drive peaks on the RAID). Curious, I though, so I started digging. Firs, I limited the torrent to 100K up and 100K down, didn't make a difference.

It seems that uTorrent admits to the reads in the disk activity in the statistics section. After using third party software, it looked like it was reading the files from the disk at a high rate. It was the files from the (only) active torrent, at about 12-20 MB per file (4MB increments). I waited until the reads equaled the downloaded section, then the selected section, then the whole torrent (48G). Keeps going at it.

What is it doing with all the data? Why is it re-reading everything repeatedly? 100M/s means the system is slow to do anything. It didn't use to do this not long ago, I was able to play just fine with uTorrent in the background.

As I type this, M/s varies in round (4M) increments, 16, 20, 32 M/s, so it looks like it reads 4M chunks, just like it was serving or hash checking the files, except it keeps doing it on files it already has, and the ones that it is still downloading.

Stopping the torrent kills the activity. I did a manual re-check of the torrent hashes and the bulk of the disk activity is attributed to system rather than uTorrent, so it looks like it's not hash checking that is the culprit.

By the time I finished this post, uTorrent read 160 GB from my disk.

Any hints? Settings are pretty much defaults.

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Found it, it was because of read cache. As uTorrent was uploading, read cache was thrashing (big torrent, assumingly with random reads) and the cache was reading like crazy. Disabled cached reading and cached reads lowered to zero, and disk reads went up by the same number of reads, except disk reads are 16K on average, cache reads were 4M. By the time I found this, uTorrent has read 5.4 Tb from disk.

So, you know, if this happens, it's the read cache.

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