On Thu, May 14, 2020 at 07:52:41AM +1000, Dave Chinner wrote: > On Wed, May 13, 2020 at 08:09:59AM -0400, Brian Foster wrote: > > On Wed, May 13, 2020 at 09:36:27AM +1000, Dave Chinner wrote: > > > On Tue, May 12, 2020 at 10:05:44AM -0400, Brian Foster wrote: > > > > Particularly as it relates to percpu functionality. Does > > > > the window scale with cpu count, for example? It might not matter either > > > > > > Not really. We need a thundering herd to cause issues, and this > > > occurs after formatting an item so we won't get a huge thundering > > > herd even when lots of threads block on the xc_ctx_lock waiting for > > > a push to complete. > > > > > > > It would be nice to have some debug code somewhere that somehow or > > another asserts or warns if the CIL reservation exceeds some > > insane/unexpected heuristic based on the current size of the context. I > > don't know what that code or heuristic looks like (i.e. multiple factors > > of the ctx size?) so I'm obviously handwaving. Just something to think > > about if we can come up with a way to accomplish that opportunistically. > > I don't think there is a reliable mechanism that can be used here. > At one end of the scale we have the valid case of a synchronous > inode modification on a log with a 256k stripe unit. So it's valid > to have a CIL reservation of ~550kB for a single item that consumes > ~700 bytes of log space. > > OTOH, we might be freeing extents on a massively fragmented file and > filesystem, so we're pushing 200kB+ transactions into the CIL for > every rolling transaction. On a filesystem with a 512 byte log > sector size and no LSU, the CIL reservations are dwarfed by the > actual metadata being logged... > > I'd suggest that looking at the ungrant trace for the CIL ticket > once it has committed will tell us exactly how much the reservation > was over-estimated, as the unused portion of the reservation will be > returned to the reserve grant head at this point in time. Typical for this workload is a CIl ticket that looks like this at ungrant time: t_curr_res 13408 t_unit_res 231100 t_curr_res 9240 t_unit_res 140724 t_curr_res 46284 t_unit_res 263964 t_curr_res 29780 t_unit_res 190020 t_curr_res 38044 t_unit_res 342016 t_curr_res 21636 t_unit_res 321476 t_curr_res 21576 t_unit_res 263964 t_curr_res 42200 t_unit_res 411852 t_curr_res 21636 t_unit_res 292720 t_curr_res 62740 t_unit_res 514552 t_curr_res 17456 t_unit_res 284504 t_curr_res 29852 t_unit_res 411852 t_curr_res 13384 t_unit_res 206452 t_curr_res 70956 t_unit_res 518660 t_curr_res 70908 t_unit_res 333800 t_curr_res 50404 t_unit_res 518660 t_curr_res 17480 t_unit_res 321476 t_curr_res 33948 t_unit_res 436500 t_curr_res 17492 t_unit_res 317368 t_curr_res 50392 t_unit_res 489904 t_curr_res 13360 t_unit_res 325584 t_curr_res 66812 t_unit_res 506336 t_curr_res 33924 t_unit_res 366664 t_curr_res 70932 t_unit_res 551524 t_curr_res 29852 t_unit_res 374880 t_curr_res 25720 t_unit_res 494012 t_curr_res 42152 t_unit_res 506336 t_curr_res 21684 t_unit_res 543308 t_curr_res 29840 t_unit_res 440608 t_curr_res 46320 t_unit_res 551524 t_curr_res 21624 t_unit_res 387204 t_curr_res 29840 t_unit_res 522768 So we are looking at a reservation of up to 500KB, and typically using all but a few 10s of KB of it. I'll use this as the ballpark for the lockless code. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
