On 04/14/2010 09:43 PM, Grant Grundler wrote:
In 2009, While running "cache read" performance test of drives behind
SII PMP we encountered a "all 5 drives" timeout on more than 30% of the
machines under test. This patch reduces the rate by a factor of about 70.
Low enough that we didn't care to further investigate the issue.
Performance impact with any sort of "normal" use was ~2%+ CPU and less
than 1% throughput degradation. Worst case impact (cached read) was
6% IOPS reduction. This is with NCQ off (q=1) but I believe FIS based
switching enabled in the SATA driver.
The patch disables "Early ACK" in the 3726 port multiplier.
"Early ACK" is issued when device sends a FIS to the host (via PMP)
and the PMP sends an ACK immediately back to the device - well before
the host gets the response. Under worst case IOPs load (cached read
test) and more than 2 PMPs connected to a 4-port SATA controller,
I suspect the time to service all of the PMPs is exceeding the PMPs
ability to keep track of outstanding FIS it owes the Host. Reducing
the number of PMPs to 2 (or 1) reduces the frequency by several orders
of magnitude. Kudos to Gwendal for initial debugging of this issue.
[Any errors in the description are mine, not his.]
Patch is currently in production on Google servers.
Signed-off-by: Grant Grundler<grundler@xxxxxxxxxx>
Signed-off-by: Gwendal Grignou<gwendal@xxxxxxxxxx>
Acked-by: Tejun Heo<tj@xxxxxxxxxx>
---
v2: dropped references to 4726 since I didn't test 4726,
moved register definition directly into libata-pmp.c, and
expanded the comment in the code to summarize the above description.
Code below is white space mangled. Please use attached file.
applied manually, please figure out a way to send patches that can be
applied directly using the standard automated tools (== git am) that
everyone uses
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