On Tue, 20 Aug 2013, Frank Mayhar wrote: > On Tue, 2013-08-20 at 18:24 -0400, Mike Snitzer wrote: > > Mikulas' point is that you cannot reduce the size to smaller than 1. > > And aside from rq-based DM, 1 is sufficient to allow for forward > > progress even when memory is completely consumed. > > > > A patch that simply changes them to 1 but makes the rq-based DM > > mempool's size configurable should actually be fine. > > So you're saying that I should submit a patch to drop the pool size for > BIO_BASED to 1 and make the pool size for REQUEST_BASED configurable? > At the moment, in dm.c the former is hardcoded to 16 and the latter is > set via MIN_IOS (currently 256). There's also io_pool, a slab pool, > which is also set via MIN_IOS. In case of request-based io, yes, submit a patch that makes it configurable. Regarding bio-based io - there is: pools->io_pool = mempool_create_slab_pool(MIN_IOS, cachep); allocating a pool of 256 entries. I think it could be reduced to 16 for bio based devices, like other pools. As for reducing 16 further down to 1 - do you have a setup where it really helps? Please describe your system - the amount of memory, the number of dm devices, and how much memory is saved by reducing the mempools from 16 to 1. > How does this relate to the rest of the DM modules? Mpath also sets > MIN_IOS to 256 and creates a slab pool from that, and there are a number > of hardcoded constants in dm-io (MIN_IOS and MIN_BIOS), dm-snap > (MIN_IOS), In dm-snap you shouldn't reduce it because it can cause failure. > dm-crypt (MIN_IOS and MIN_POOL_PAGES), dm-bufio > (DM_BUFIO_HASH_BITS, which is allocated via vmalloc per client) and dm-bufio hash could be reduced - one possibility is to auto-tune it based on device size, another possibility is to allocate shared hash table for all devices. > dm-verity (DM_VERITY_MEMPOOL_SIZE, which is allocated per device). > > For the most part I can't imagine that people will want to change these > from their defaults, but when someone does need to change one of these, > they need to do so badly and there's currently no good way to do that > besides hacking the source and building a new kernel. > > By the way, I do appreciate the advice. I'm just trying to clear up > confusion on my part, make sure that our needs are met and, while I'm at > it, make things a bit better for those who come after me. > -- > Frank Mayhar > 310-460-4042 Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
