On Tue, 22 Jan 2013 10:30:28 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote: > swap_lock is heavily contended when I test swap to 3 fast SSD (even slightly > slower than swap to 2 such SSD). The main contention comes from > swap_info_get(). This patch tries to fix the gap with adding a new > per-partition lock. > > global data like nr_swapfiles, total_swap_pages, least_priority and swap_list are > still protected by swap_lock. > > nr_swap_pages is an atomic now, it can be changed without swap_lock. In theory, > it's possible get_swap_page() finds no swap pages but actually there are free > swap pages. But sounds not a big problem. > > accessing partition specific data (like scan_swap_map and so on) is only > protected by swap_info_struct.lock. > > Changing swap_info_struct.flags need hold swap_lock and swap_info_struct.lock, > because scan_scan_map() will check it. read the flags is ok with either the > locks hold. > > If both swap_lock and swap_info_struct.lock must be hold, we always hold the > former first to avoid deadlock. > > swap_entry_free() can change swap_list. To delete that code, we add a new > highest_priority_index. Whenever get_swap_page() is called, we check it. If > it's valid, we use it. > > It's a pitty get_swap_page() still holds swap_lock(). But in practice, > swap_lock() isn't heavily contended in my test with this patch (or I can say > there are other much more heavier bottlenecks like TLB flush). And BTW, looks > get_swap_page() doesn't really need the lock. We never free swap_info[] and we > check SWAP_WRITEOK flag. The only risk without the lock is we could swapout to > some low priority swap, but we can quickly recover after several rounds of > swap, so sounds not a big deal to me. But I'd prefer to fix this if it's a real I had to move a few things around due to changes in drivers/staging/zcache/. Do you have any performance testing results for this patch? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>