Re: [PATCH 0/2] mm,swap: skip swap readahead for instant IO (like zswap)
On Mon, 2020-10-05 at 13:32 -0400, Rik van Riel wrote: > On Tue, 2020-09-22 at 10:12 -0700, Andrew Morton wrote: > > On Mon, 21 Sep 2020 22:01:46 -0400 Rik van Riel > > wrote: > > Any quantitative testing results? > > I have test results with a real workload now. > > Without this patch, enabling zswap results in about an > 8% increase in p99 request latency. With these patches, > the latency penalty for enabling zswap is under 1%. Never mind that. On larger tests the effect seems to disappear, probably because the logic in __swapin_nr_pages() already reduces the number of pages read ahead to 2 on workloads with lots of random access. That reduces the latency effects observed. Now we might still see some memory waste due to decompressing pages we don't need, but I have not seen any real effects from that yet, either. I think it may be time to focus on a larger memory waste with zswap: leaving the compressed copy of memory around when we decompress the memory at swapin time. More aggressively freeing the compressed memory will probably buy us more than reducing readahead. -- All Rights Reversed. signature.asc Description: This is a digitally signed message part
Re: [PATCH 0/2] mm,swap: skip swap readahead for instant IO (like zswap)
On Tue, 2020-09-22 at 10:12 -0700, Andrew Morton wrote: > On Mon, 21 Sep 2020 22:01:46 -0400 Rik van Riel > wrote: > > > Both with frontswap/zswap, and with some extremely fast IO devices, > > swap IO will be done before the "asynchronous" swap_readpage() call > > has returned. > > > > In that case, doing swap readahead only wastes memory, increases > > latency, and increases the chances of needing to evict something > > more > > useful from memory. In that case, just skip swap readahead. > > Any quantitative testing results? I have test results with a real workload now. Without this patch, enabling zswap results in about an 8% increase in p99 request latency. With these patches, the latency penalty for enabling zswap is under 1%. Enabling zswap allows us to give the main workload a little more memory, since the spikes in memory demand caused by things like system management software no longer cause large latency issues. -- All Rights Reversed. signature.asc Description: This is a digitally signed message part
Re: [PATCH 0/2] mm,swap: skip swap readahead for instant IO (like zswap)
On Mon, 21 Sep 2020 22:01:46 -0400 Rik van Riel wrote: > Both with frontswap/zswap, and with some extremely fast IO devices, > swap IO will be done before the "asynchronous" swap_readpage() call > has returned. > > In that case, doing swap readahead only wastes memory, increases > latency, and increases the chances of needing to evict something more > useful from memory. In that case, just skip swap readahead. Any quantitative testing results?
[PATCH 0/2] mm,swap: skip swap readahead for instant IO (like zswap)
Both with frontswap/zswap, and with some extremely fast IO devices, swap IO will be done before the "asynchronous" swap_readpage() call has returned. In that case, doing swap readahead only wastes memory, increases latency, and increases the chances of needing to evict something more useful from memory. In that case, just skip swap readahead.
