Messages in this thread |  | | Date | Fri, 12 Oct 2001 22:09:37 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: Linux 2.4.10-ac11: swapoff frees memory + swap? |
| |
On Thu, 11 Oct 2001, Matthias Andree wrote: > > It swaps blazingly fast, but one strange observation. After the efenced > application had died at approx 300 MB in RAM and 180 MB of swap, I had > somewhat around 130 MB in swap and like 250 MB "USED+SHAR" as per > xosview. That looked too high a number, so I did swapoff -av, and after > that, I had 90 MB used. The swapoff was rather fast, compared with older > 2.4.x vanilla kernels. > > It may well be a cosmetic issue, but it's irritating that switching the > swap off looks like freeing main memory as well, one might expect pages > are swapped back into RAM, so USED increases.
That's because when your application exits, zap_pte_range frees page and swap for the present ptes, but only swap_free for the non-present ptes: sometimes that brings the swap count down to 1 (still used), but that 1 corresponds to page remaining in the swap cache which could now be freed.
There's no lookup in that case, intentionally. A patch was posted a few months ago to do so, but Linus preferred not to add such unmap overhead. Indeed, for a while we didn't even free swap for the present ptes, but a number of problems arose from that (maybe now fixed in other ways, but I don't think we dare to reopen that wormcan).
In due course, when memory pressure demands, reclaim_page (Alan+Rik) or shrink_cache (Linus+Andrea) will discover those pages and make them available. Or, as you found, swapoff will free them: one of the reasons swapoff is now faster is that it no longers searches mms in that case.
Hugh
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
|  |