>>>>> "Coly" == Coly Li <colyli@xxxxxxx> writes: Coly> On 2019/12/12 11:48 上午, John Stoffel wrote: >>>>>>> "Sasha" == Sasha Levin <sashal@xxxxxxxxxx> writes: >> Sasha> From: Coly Li <colyli@xxxxxxx> Sasha> [ Upstream commit 9fcc34b1a6dd4b8e5337e2b6ef45e428897eca6b ] >> Sasha> In bch_mca_scan(), the number of shrinking btree node is calculated Sasha> by code like this, Sasha> unsigned long nr = sc->nr_to_scan; >> Sasha> nr /= c->btree_pages; Sasha> nr = min_t(unsigned long, nr, mca_can_free(c)); Sasha> variable sc->nr_to_scan is number of objects (here is bcache B+tree Sasha> nodes' number) to shrink, and pointer variable sc is sent from memory Sasha> management code as parametr of a callback. >> Sasha> If sc->nr_to_scan is smaller than c->btree_pages, after the above Sasha> calculation, variable 'nr' will be 0 and nothing will be shrunk. It is Sasha> frequeently observed that only 1 or 2 is set to sc->nr_to_scan and make Sasha> nr to be zero. Then bch_mca_scan() will do nothing more then acquiring Sasha> and releasing mutex c->bucket_lock. >> Sasha> This patch checkes whether nr is 0 after the above calculation, if 0 Sasha> is the result then set 1 to variable 'n'. Then at least bch_mca_scan() Sasha> will try to shrink a single B+tree node. >> Sasha> nr /= c->btree_pages; Sasha> + if (nr == 0) Sasha> + nr = 1; >> >> >> Wouldn't it be even more clear with: >> >> nr /= c->bree_pages || 1; >> >> instead? Coly> No, it is not more clear. At least to me, I may confuse does it mean, Coly> - nr = (nr / c->btree_pages) || 1; Coly> - or nr = nr / (c->btree_pages || 1) Coly> If I don't check C manual, I am not able to tell the correct Coly> calculate at first time. You're right, it's not quite as clear, it needs proper parenthesis. But maybe instead of a (possibly) expensive division all the time, why not just shift and assume you have it shrink a node, or try to. I honestly haven't looked closely enough at the code to figure out the best shift to use here. But isn't this calculation wrong anyway? If you have lots of c->bree_pages, wouldn't you want to do more freeing? I'd need to read the code better, but I'm heading to bed now. Sorry. John
