On Thu, 26 Jun 2014 13:44:12 +0200 Sebastien Buisson <sebastien.buisson@xxxxxxxx> wrote: > > Le 26/06/2014 00:16, Andrew Morton a __crit : > > On Tue, 24 Jun 2014 17:52:00 +0200 Sebastien Buisson <sebastien.buisson@xxxxxxxx> wrote: > > > >> Allow increasing the buffer-head per-CPU LRU size to allow efficient > >> filesystem operations that access many blocks for each transaction. > >> For example, creating a file in a large ext4 directory with quota > >> enabled will accesses multiple buffer heads and will overflow the LRU > >> at the default 8-block LRU size: > >> > >> * parent directory inode table block (ctime, nlinks for subdirs) > >> * new inode bitmap > >> * inode table block > >> * 2 quota blocks > >> * directory leaf block (not reused, but pollutes one cache entry) > >> * 2 levels htree blocks (only one is reused, other pollutes cache) > >> * 2 levels indirect/index blocks (only one is reused) > >> > >> Make this tuning be a kernel parameter 'bh_lru_size'. > > > > I don't think it's a great idea to make this a boot-time tunable. It's > > going to take a ton of work by each and every kernel > > user/installer/distributor to work out what is the best setting for > > them. And the differences will be pretty small anyway. And we didn't > > provide them with any documentation to help them even get started with > > the project. > > > > I am sorry, I meant to leave the default bh_lru_size as is, ie set to 8 > (instead of 16 in my proposed patch). That way, kernel users and > integrators of all kind would not have to bother about the new boot-time > tunable, and could change nothing and stay with the same value as they > did before. > > At the same time, advanced users like those playing with Lustre would > have the ability to tune the buffer-head per-CPU LRU size without the > need to recompile the kernel. > > Does it sound better? Mutter. Maybe. But is there any downside to increasing BH_LRU_SIZE to 8? Or, more accurately, does that downside outweight the upside? That "8" was pulled out of a hat 12 years ago and I don't think anyone has before done any serious investigation into tuning it. Maybe 16 is just a better setting? -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
