Walt Mankowski on 16 Apr 2015 07:42:39 -0700

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Re: [PLUG] swappiness and ssd

On Thu, Apr 16, 2015 at 10:01:55AM -0400, K.S. Bhaskar wrote:
> The default swappiness on Ubuntu systems is 60, and presumably takes into
> consideration the relative performance of writes to and reads from the swap
> file / partition.  The ratio of write performance to read performance would
> be higher for SSDs than rotating media, since SSD reads are truly random
> whereas writes are in pages (in rotating media, seek and latency times
> would tend to apply to both).  Therefore, it seems to me that if swap is
> located on SSD, we should increase swappiness, to encourage less frequently
> used pages to be written out, since they can be brought back in very fast
> when needed.  Does that seem reasonable or am I missing something?  Thanks.

I'm not sure why you think that SSD reads are "truly random".  First,
many reads will be buffered, so you'll still be reading in pages at a
time instead of bytes at a time.  Also, keep in mind the kernel is in
the way of all your reads.  It can cache frequently accessed data, and
also reorder reads so that they're performed more efficiently.


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