This happens more and more often in crosperf nightly tests. The error looks like :

Page_cycler_v2.typical_25 FAILED Extra data: line 28707 column 1 - line 28865 column 1 (char 958792 - 963466

I finally managed to get a good v.s. bad comparison between runs. The attached json results belong to speedometer, however, there are garbages in the end of bad.json. The garbage looks like a part of the result of smoothness.tough_webgl_cases.

Deleted: good.json 9.2 KB

good.json 9.2 KB View Download

Deleted: bad.json 342 KB

bad.json 342 KB View Download

mmm. this is pretty bad! 
thanks for taking ownership.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/e0cbc35b4bf89398ba5f35c1b8f15b7a3d6bc596

commit e0cbc35b4bf89398ba5f35c1b8f15b7a3d6bc596
Author: Ting-Yuan Huang <laszio@chromium.org>
Date: Tue May 02 02:18:18 2017

telemetry_Crosperf: fix a concurrency issue

crosperf can run multiple telemetry tests simultaneously. By default,
the results are written to the same directory and collision happens.

BUG= chromium:678340 
TEST=crosperf runs successfully

Change-Id: Ia1d8dec730bf4f669c140c6a5bcbeb89c20f7be4
Reviewed-on: https://chromium-review.googlesource.com/490639
Commit-Ready: Ting-Yuan Huang <laszio@chromium.org>
Tested-by: Ting-Yuan Huang <laszio@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>

[modify] https://crrev.com/e0cbc35b4bf89398ba5f35c1b8f15b7a3d6bc596/server/site_tests/telemetry_Crosperf/telemetry_Crosperf.py

No more such errors in nightly perf tests.