Messages in this thread | | | Subject | Re: [v3 PATCH 1/2] mm: swap: check if swap backing device is congested or not | From | Yang Shi <> | Date | Fri, 28 Dec 2018 17:41:01 -0800 |
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On 12/28/18 4:42 PM, Andrew Morton wrote: > On Sat, 22 Dec 2018 05:40:19 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote: > >> Swap readahead would read in a few pages regardless if the underlying >> device is busy or not. It may incur long waiting time if the device is >> congested, and it may also exacerbate the congestion. >> >> Use inode_read_congested() to check if the underlying device is busy or >> not like what file page readahead does. Get inode from swap_info_struct. >> Although we can add inode information in swap_address_space >> (address_space->host), it may lead some unexpected side effect, i.e. >> it may break mapping_cap_account_dirty(). Using inode from >> swap_info_struct seems simple and good enough. >> >> Just does the check in vma_cluster_readahead() since >> swap_vma_readahead() is just used for non-rotational device which >> much less likely has congestion than traditional HDD. >> >> Although swap slots may be consecutive on swap partition, it still may be >> fragmented on swap file. This check would help to reduce excessive stall >> for such case. > Some words about the observed effects of the patch would be more than > appropriate!
Yes, sure. Actually, this could reduce the latency long tail of do_swap_page() on a congested system.
The test on my virtual machine with emulated HDD shows:
Without swap congestion check: page_fault1_thr-1490 [023] 129.311706: funcgraph_entry: # 57377.796 us | do_swap_page(); page_fault1_thr-1490 [023] 129.369103: funcgraph_entry: 5.642 us | do_swap_page(); page_fault1_thr-1490 [023] 129.369119: funcgraph_entry: # 1289.592 us | do_swap_page(); page_fault1_thr-1490 [023] 129.370411: funcgraph_entry: 4.957 us | do_swap_page(); page_fault1_thr-1490 [023] 129.370419: funcgraph_entry: 1.940 us | do_swap_page(); page_fault1_thr-1490 [023] 129.378847: funcgraph_entry: # 1411.385 us | do_swap_page(); page_fault1_thr-1490 [023] 129.380262: funcgraph_entry: 3.916 us | do_swap_page(); page_fault1_thr-1490 [023] 129.380275: funcgraph_entry: # 4287.751 us | do_swap_page();
With swap congestion check: runtest.py-1417 [020] 301.925911: funcgraph_entry: # 9870.146 us | do_swap_page(); runtest.py-1417 [020] 301.935785: funcgraph_entry: 9.802 us | do_swap_page(); runtest.py-1417 [020] 301.935799: funcgraph_entry: 3.551 us | do_swap_page(); runtest.py-1417 [020] 301.935806: funcgraph_entry: 2.142 us | do_swap_page(); runtest.py-1417 [020] 301.935853: funcgraph_entry: 6.938 us | do_swap_page(); runtest.py-1417 [020] 301.935864: funcgraph_entry: 3.765 us | do_swap_page(); runtest.py-1417 [020] 301.935871: funcgraph_entry: 3.600 us | do_swap_page(); runtest.py-1417 [020] 301.935878: funcgraph_entry: 7.202 us | do_swap_page();
The long tail latency (>1000us) is reduced significantly.
BTW, do you need I resend the patch with the above information appended into the commit log?
Thanks, Yang
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