> On Apr 12, 2024, at 1:44 PM, Jianfeng Wang <jianfeng.w.wang@xxxxxxxxxx> wrote: > > On 4/12/24 1:20 PM, Vlastimil Babka wrote: >> On 4/12/24 7:29 PM, Jianfeng Wang wrote: >>> >>> >>> On 4/12/24 12:48 AM, Vlastimil Babka wrote: >>>> On 4/11/24 7:02 PM, Christoph Lameter (Ampere) wrote: >>>>> On Thu, 11 Apr 2024, Jianfeng Wang wrote: >>>>> >>>>>> So, the fix is to limit the number of slabs to scan in >>>>>> count_partial(), and output an approximated result if the list is too >>>>>> long. Default to 10000 which should be enough for most sane cases. >>>>> >>>>> >>>>> That is a creative approach. The problem though is that objects on the >>>>> partial lists are kind of sorted. The partial slabs with only a few >>>>> objects available are at the start of the list so that allocations cause >>>>> them to be removed from the partial list fast. Full slabs do not need to >>>>> be tracked on any list. >>>>> >>>>> The partial slabs with few objects are put at the end of the partial list >>>>> in the hope that the few objects remaining will also be freed which would >>>>> allow the freeing of the slab folio. >>>>> >>>>> So the object density may be higher at the beginning of the list. >>>>> >>>>> kmem_cache_shrink() will explicitly sort the partial lists to put the >>>>> partial pages in that order. >>>>> Realized that I’d do "echo 1 > /sys/kernel/slab/dentry/shrink” to sort the list explicitly. After that, the numbers become: N = 10000 -> diff = 7.1 % N = 20000 -> diff = 5.7 % N = 25000 -> diff = 5.4 % So, expecting ~5-7% difference after shrinking. >>>>> Can you run some tests showing the difference between the estimation and >>>>> the real count? >>> >>> Yes. >>> On a server with one NUMA node, I create a case that uses many dentry objects. >> >> Could you describe in more detail how do you make dentry cache to grow such >> a large partial slabs list? Thanks. >> > > I utilized the fact that creating a folder will create a new dentry object; > deleting a folder will delete all its sub-folder's dentry objects. > > Then, I started to create N folders, while each folder has M empty sub-folders. > Assuming that these operations would consume a large number of dentry > objects in the sequential order. Their slabs were very likely to be full slabs. > After all folders were created, I deleted a subset of the N folders (i.e., > one out of every two folders). This would create many holes, which turned a > subset of full slabs into partial slabs.
