Temporary files: RAM or disk?

Posted Jun 6, 2012 15:24 UTC (Wed) by nybble41 (subscriber, #55106)
In reply to: Temporary files: RAM or disk? by Serge
Parent article: Temporary files: RAM or disk?

> But reading from swap means that your RAM is full, and in order to read a page from swap you must first write another page there. I.e. sequential read from ext3 turns into random write+read from swap.

_Writing_ to swap means that your RAM is full (possibly including things like clean cache which are currently higher priority, but could be dropped at need). _Reading_ from swap implies only that something previously written to swap is needed in RAM again. There could be any amount of free space at that point. Even if RAM does happen to be full, the kernel can still drop clean data from the cache to make room, just as with reading from ext3.

Temporary files: RAM or disk?

Posted Jun 6, 2012 17:43 UTC (Wed) by dgm (subscriber, #49227) [Link]

Yes, merely reading from swap doesn't imply that your RAM is full. What is true is that _when_ your RAM is full (notice that I don't say "if") it _may_ imply a write to swap, depending in how dirty the page cache is. The problem is, tmpfs is a factor that contributes a lot to pollute the page cache. Temporary files are created to be written and then re-read in short, so all pages used by tmpfs are expected to be dirty.

All of this is of no consequence on system startup, when the page cache is mostly clean. Once the system has been up for a while, though... I think a few tests have to be done.