Re: hdparm -t yields incorrect timings when Intel hyperthreading is enabled
On Fri, 09 May 2014, Paul Ausbeck wrote:
> I've actually done dummy file reads and writes previously. Well
> actually just writes. And they go at full speed, no matter what
> hparm says. For example, your example, works at full speed:
> dd if=/dev/zero of=somefile bs=10M count=100 ;
You have to call "sync", otherwise you've measured the time to write to
the buffer-cache...
That said, try this:
strace -o /tmp/strace.txt -ttt -T hdparm -t (rest of hdparm command).
And check if the timings of the syscalls traced give you any insight on
where things are going wrong. Compare HT enabled with HT disabled.
> but I wasn't able to find a way to purge the disk cache before I got sidetracked. Perhaps you know of a magic incantation for that?
Yes. You can drop all caches if you issue, as root:
sync ; echo 1 > /proc/sys/vm/drop_caches
According to https://www.kernel.org/doc/Documentation/sysctl/vm.txt
drop_caches
-----------
Writing to this will cause the kernel to drop clean caches, as well as
reclaimable slab objects like dentries and inodes. Once dropped, their
memory becomes free.
To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
To free reclaimable slab objects (includes dentries and inodes):
echo 2 > /proc/sys/vm/drop_caches
To free slab objects and pagecache:
echo 3 > /proc/sys/vm/drop_caches
This is a non-destructive operation and will not free any dirty objects.
To increase the number of objects freed by this operation, the user may run
`sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the
number of dirty objects on the system and create more candidates to be
dropped.
This file is not a means to control the growth of the various kernel caches
(inodes, dentries, pagecache, etc...) These objects are automatically
reclaimed by the kernel when memory is needed elsewhere on the system.
Use of this file can cause performance problems. Since it discards cached
objects, it may cost a significant amount of I/O and CPU to recreate the
dropped objects, especially if they were under heavy use. Because of this,
use outside of a testing or debugging environment is not recommended.
You may see informational messages in your kernel log when this file is
used:
cat (1234): drop_caches: 3
These are informational only. They do not mean that anything is wrong
with your system. To disable them, echo 4 (bit 3) into drop_caches.
--
"One disk to rule them all, One disk to find them. One disk to bring
them all and in the darkness grind them. In the Land of Redmond
where the shadows lie." -- The Silicon Valley Tarot
Henrique Holschuh
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